Those who will be punished in hell hereafter, if they enter Jannah would they enjoy it? Punishment would be very bad and maybe very long, wouldn't they feel regrets and loss that many others entered before them?.
Yes, they will enjoy it as everyone in Jannah enjoys it. Any traces of their previous punishment will be removed, and they will get their stance in Jannah that they will be content with. Furthermore, they will hold no resentment to anyone else in Jannah.
Everyone in Jannah will be given what will make them happy and content, to varying degrees according to their deeds, but all are on a positive scale of happiness and content. The Prophet ﷺ said that dwellers of Jannah will be given what no one else will be given, then Allah will shed on them his pleasure forever:
عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، أَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ: إِنَّ اللَّهَ يَقُولُ لأَهْلِ الْجَنَّةِ يَا أَهْلَ الْجَنَّةِ. فَيَقُولُونَ لَبَّيْكَ رَبَّنَا وَسَعْدَيْكَ وَالْخَيْرُ فِي يَدَيْكَ. فَيَقُولُ هَلْ رَضِيتُمْ فَيَقُولُونَ وَمَا لَنَا لاَ نَرْضَى يَا رَبِّ وَقَدْ أَعْطَيْتَنَا مَا لَمْ تُعْطِ أَحَدًا مِنْ خَلْقِكَ فَيَقُولُ أَلاَ أُعْطِيكُمْ أَفْضَلَ مِنْ ذَلِكَ فَيَقُولُونَ يَا رَبِّ وَأَىُّ شَىْءٍ أَفْضَلُ مِنْ ذَلِكَ فَيَقُولُ أُحِلُّ عَلَيْكُمْ رِضْوَانِي فَلاَ أَسْخَطُ عَلَيْكُمْ بَعْدَهُ أَبَدًا
Abu Sa'id al-Khudri reported that Allah's Apostle (ﷺ) said that Allah would say to the inmates of Paradise: O, Dwellers of Paradise, and they would say in response: At thy service and pleasure, our Lord, the good is in Thy Hand. He (the Lord) would say: Are you well pleased now? They would say: Why should we not be pleased, O Lord, when Thou hast given us what Thou hast not given to any of Thy creatures? He would, however, say: May I not give you (something) even more excellent than that? And they would say: O Lord, what thing can be more excellent than this? And He would say: I shall cause My pleasure to alight upon you and I shall never be afterward annoyed with you.
The ultimate pleasure is to be able to see Allah when one is in Jannah:
وُجُوهٌ يَوْمَئِذٍ نَّاضِرَةٌ إِلَىٰ رَبِّهَا نَاظِرَةٌ
[Some] faces, that Day, will be radiant, looking at their Lord.
In Jannah, there is no resentment in the hearts of its dwellers as Allah informs us in the Qur'an. Whoever enters Jannah after punishment in Jahannam will adopt the attributes of the people of Jannah and will bear no resentment:
وَنَزَعْنَا مَا فِي صُدُورِهِم مِّنْ غِلٍّ إِخْوَانًا عَلَىٰ سُرُرٍ مُّتَقَابِلِينَ
And We will remove whatever is in their breasts of resentment, [so they will be] brothers, on thrones facing each other.
All traces of punishment will be removed as they get immersed in the River of Life after emerging from Jahannam and they will grow into healthy human beings ready to be admitted into Jannah:
عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ: يُدْخِلُ اللَّهُ أَهْلَ الْجَنَّةِ الْجَنَّةَ يُدْخِلُ مَنْ يَشَاءُ بِرَحْمَتَهِ وَيُدْخِلُ أَهْلَ النَّارِ النَّارَ ثُمَّ يَقُولُ انْظُرُوا مَنْ وَجَدْتُمْ فِي قَلْبِهِ مِثْقَالَ حَبَّةٍ مِنْ خَرْدَلٍ مِنْ إِيمَانٍ فَأَخْرِجُوهُ. فَيُخْرَجُونَ مِنْهَا حُمَمًا قَدِ امْتَحَشُوا. فَيُلْقَوْنَ فِي نَهْرِ الْحَيَاةِ أَوِ الْحَيَا فَيَنْبُتُونَ فِيهِ كَمَا تَنْبُتُ الْحِبَّةُ إِلَى جَانِبِ السَّيْلِ أَلَمْ تَرَوْهَا كَيْفَ تَخْرُجُ صَفْرَاءَ مُلْتَوِيَةً
Abu Sa'id al-Khudri reported: Verily the Messenger of Allah (ﷺ) said: Allah will admit into Paradise those deserving of Paradise, and He will admit whom He wishes out of His Mercy, and admit those condemned to Hell into the Fire (of Hell). He would then say: See, he whom you find having as much faith in his heart as a grain of mustard, bring him out. They will then be brought out burned and turned to charcoal, and would be cast into the River of Life, and they would sprout as does a seed in the silt carried away by flood. Have you not seen that it comes out yellow (fresh) and intertwined?
To further illustrate the point, let's look at the last person emerging out of Jahannam and being admitted into Jannah, and what he gets in Jannah. From the hadith narrated in Sahih Muslim, Book 1, Hadith 370 under the authority of 'Abdullah ibn Mas'ud that the Prophet ﷺ said:
آخِرُ مَنْ يَدْخُلُ الْجَنَّةَ رَجُلٌ فَهُوَ يَمْشِي مَرَّةً وَيَكْبُو مَرَّةً وَتَسْفَعُهُ النَّارُ مَرَّةً فَإِذَا مَا جَاوَزَهَا الْتَفَتَ إِلَيْهَا فَقَالَ تَبَارَكَ الَّذِي نَجَّانِي مِنْكِ لَقَدْ أَعْطَانِيَ اللَّهُ شَيْئًا مَا أَعْطَاهُ أَحَدًا مِنَ الأَوَّلِينَ وَالآخِرِينَ
The last to enter Paradise would be a man who would walk once and stumble once and be burnt by the Fire once. Then when he gets beyond it, he will turn to it and say: "Blessed is He Who has saved me from thee. Allah has given me something He has not given to any one of those in earlier or later times."
As you can see, this man will assume that he will have been given more than everyone else just by being saved from Jahannam. He will assume that being saved from Jahannam is the greatest blessing of all.
فَتُرْفَعُ لَهُ شَجَرَةٌ فَيَقُولُ أَىْ رَبِّ أَدْنِنِي مِنْ هَذِهِ الشَّجَرَةِ فَلأَسْتَظِلَّ بِظِلِّهَا وَأَشْرَبَ مِنْ مَائِهَا فَيَقُولُ اللَّهُ عَزَّ وَجَلَّ يَا ابْنَ آدَمَ لَعَلِّي إِنْ أَعْطَيْتُكَهَا سَأَلْتَنِي غَيْرَهَا فَيَقُولُ لاَ يَا رَبِّ وَيُعَاهِدُهُ أَنْ لاَ يَسْأَلَهُ غَيْرَهَا وَرَبُّهُ يَعْذِرُهُ لأَنَّهُ يَرَى مَا لاَ صَبْرَ لَهُ عَلَيْهِ فَيُدْنِيهِ مِنْهَا فَيَسْتَظِلُّ بِظِلِّهَا وَيَشْرَبُ مِنْ مَائِهَا
Then a tree would be raised up for him and he will say: "O my Lord I bring me near this tree so that I may take shelter in its shade and drink of its water." Allah, the Exalted and Great, would say: "O son of Adam, if I grant you this, you will ask Me for something else." He would say: "No, my Lord." And he would promise Him that he would not ask for anything else. His Lord would excuse him because he sees what he cannot help desiring; so He would bring him near it, and he would take shelter in its shade and drink of its water.
A simple tree will appear to him as the ultimate blessing anyone can get; hence, he assures Allah that he would not ask for more. The tree's shade and water are all that he will dream of at this stage.
ثُمَّ تُرْفَعُ لَهُ شَجَرَةٌ هِيَ أَحْسَنُ مِنَ الأُولَى فَيَقُولُ أَىْ رَبِّ أَدْنِنِي مِنْ هَذِهِ لأَشْرَبَ مِنْ مَائِهَا وَأَسْتَظِلَّ بِظِلِّهَا لاَ أَسْأَلُكَ غَيْرَهَا فَيَقُولُ يَا ابْنَ آدَمَ أَلَمْ تُعَاهِدْنِي أَنْ لاَ تَسْأَلَنِي غَيْرَهَا فَيَقُولُ لَعَلِّي إِنْ أَدْنَيْتُكَ مِنْهَا تَسْأَلُنِي غَيْرَهَا فَيُعَاهِدُهُ أَنْ لاَ يَسْأَلَهُ غَيْرَهَا وَرَبُّهُ يَعْذِرُهُ لأَنَّهُ يَرَى مَا لاَ صَبْرَ لَهُ عَلَيْهِ فَيُدْنِيهِ مِنْهَا فَيَسْتَظِلُّ بِظِلِّهَا وَيَشْرَبُ مِنْ مَائِهَا
Afterwards a tree more beautiful than the first would be raised up before him and he would say: "O my Lord! bring me near this tree in order that I may drink of its water and take shelter in its shade and I shall not ask Thee for anything else." He (Allah) would say: "O son of Adam, if I bring you near it you may ask me for something else." He would promise Him that he would not ask for anything else. His Lord will excuse him because he would see something he cannot help desiring. So He would bring him near it and he would enjoy its shade and drink its water.
When a better tree emerges, the man wants to go there. The significance of the second tree is to show that the human nature is not to be happy with what it has. We always want more, but later on in the same hadith when the man enters Jannah, he becomes happy and content (attributes of the dwellers of Jannah).
ثُمَّ تُرْفَعُ لَهُ شَجَرَةٌ عِنْدَ بَابِ الْجَنَّةِ هِيَ أَحْسَنُ مِنَ الأُولَيَيْنِ فَيَقُولُ أَىْ رَبِّ أَدْنِنِي مِنْ هَذِهِ لأَسْتَظِلَّ بِظِلِّهَا وَأَشْرَبَ مِنْ مَائِهَا لاَ أَسْأَلُكَ غَيْرَهَا فَيَقُولُ يَا ابْنَ آدَمَ أَلَمْ تُعَاهِدْنِي أَنْ لاَ تَسْأَلَنِي غَيْرَهَا قَالَ بَلَى يَا رَبِّ هَذِهِ لاَ أَسْأَلُكَ غَيْرَهَا وَرَبُّهُ يَعْذِرُهُ لأَنَّهُ يَرَى مَا لاَ صَبْرَ لَهُ عَلَيْهَا فَيُدْنِيهِ مِنْهَا فَإِذَا أَدْنَاهُ مِنْهَا فَيَسْمَعُ أَصْوَاتَ أَهْلِ الْجَنَّةِ فَيَقُولُ أَىْ رَبِّ أَدْخِلْنِيهَا
Then a tree would be raised up for him at the gate of the Paradise, more beautiful than the first two. He would say: "O my Lord! bring me near this (tree) so that I may enjoy its shade and drink from its water. I shall not ask Thee for anything else." He (Allah) would say "O son of Adam! did you not promise Me that you would not ask Me anything else?" He would say: "Yes, my Lord, but I shall not ask Thee for anything else." His Lord would excuse him for he sees something the temptation of which he could not resist. He (Allah) would bring him near to it, and when He would bring him near it he would hear the voices of the inhabitants of the Paradise. He would say: O my Lord! admit me to it.
The third tree further illustrates the human nature of not being content (he is, to that point, not yet admitted into Jannah). As he gets closer to its gates, and he hears and sees its pleasures, he forsakes all previous promises of not asking for more and requests from Allah to be admitted into it.
فَيَقُولُ يَا ابْنَ آدَمَ مَا يَصْرِينِي مِنْكَ أَيُرْضِيكَ أَنْ أُعْطِيَكَ الدُّنْيَا وَمِثْلَهَا مَعَهَا قَالَ يَا رَبِّ أَتَسْتَهْزِئُ مِنِّي وَأَنْتَ رَبُّ الْعَالَمِينَ. فَضَحِكَ ابْنُ مَسْعُودٍ فَقَالَ أَلاَ تَسْأَلُونِّي مِمَّ أَضْحَكُ فَقَالُوا مِمَّ تَضْحَكُ قَالَ هَكَذَا ضَحِكَ رَسُولُ اللَّهِ صلى الله عليه وسلم فَقَالُوا مِمَّ تَضْحَكُ يَا رَسُولَ اللَّهِ قَالَ: مِنْ ضِحْكِ رَبِّ الْعَالَمِينَ حِينَ قَالَ أَتَسْتَهْزِئُ مِنِّي وَأَنْتَ رَبُّ الْعَالَمِينَ فَيَقُولُ إِنِّي لاَ أَسْتَهْزِئُ مِنْكَ وَلَكِنِّي عَلَى مَا أَشَاءُ قَادِرٌ
He (Allah) would say: "O son of Adam, what will bring an end to your requests to Me? Will it please you if I give you the whole world and a like one along with it?" He will say: "O my Lord! art Thou mocking me, though Thou art the Lord of the worlds?" Ibn Mas'ud laughed and asked (the hearers): "Why don't you ask me what I am laughing at?' They (then) said: "Why do you laugh?" He said: "It is in this way that the Messenger of Allah (ﷺ) laughed. They (the companions of the Holy Prophet) asked: 'Why do you laugh, Messenger of Allah?' He said: 'On account of the laugh of the Lord of the universe when he desired of Paradise and asked if He was mocking him though He is the Lord of the worlds?'" He would say: "I am not mocking you, but I have the power to do whatever I will."
At this point, Allah offers the man twice what is in this worldly life, with its vastness, rivers, trees, gold, silver, etc. He will get at least twice as much as this whole planet. Who would not be content and happy at this stage?