First of all, may Allah cure your mother, and grant her full and speedy recovery.
Second, the most effective supplication now is:
إنا لله وإنا إليه راجعون ـ حسبى الله ونعم الوكيل ـ ولا حول ولا قوة إلا
We all return to Allah one day. Sufficient for us is Allah, and [He
is] the best Disposer of affairs. There is no will or power except
Inna li Allah wa inna elaih raje'oon. Hasbi Allah wa ni'm Al-Wakeel. Wa la hawl wa la quwwata ella bi Allah.
Any calamities that befall a Muslim are one of the following:
- A reward from Allah to raise one's level in Jannah if one is patient and thankful to Allah.
- A test from Allah to see how one fares.
- A punishment from Allah for one's deeds.
To pass this test, and it is indeed a tough one, is to accept the calamity and realize that irrespective of all, we all return to Allah one day, through a terminal disease or otherwise. If one accepts his fate and seeks Allah's mercy, one will get it and more:
وَلَنَبْلُوَنَّكُم بِشَيْءٍ مِّنَ الْخَوْفِ وَالْجُوعِ وَنَقْصٍ مِّنَ الْأَمْوَالِ وَالْأَنفُسِ وَالثَّمَرَاتِ ۗ وَبَشِّرِ الصَّابِرِينَ الَّذِينَ إِذَا أَصَابَتْهُم مُّصِيبَةٌ قَالُوا إِنَّا لِلَّهِ وَإِنَّا إِلَيْهِ رَاجِعُونَ أُولَٰئِكَ عَلَيْهِمْ صَلَوَاتٌ مِّن رَّبِّهِمْ وَرَحْمَةٌ ۖ وَأُولَٰئِكَ هُمُ الْمُهْتَدُونَ
And We will surely test you with something of fear and hunger and a loss of wealth and lives and fruits, but give good tidings to the patient,who, when disaster strikes them, say, "Indeed we belong to Allah, and indeed to Him we will return." Those are the ones upon whom are blessings from their Lord and mercy. And it is those who are the [rightly] guided.
— Qur'an 2:155-157
There is a famous quote by Al-Hasan al-Basri that one should not resent calamities as one may hate a calamity in which there is one's salvation, and equally one may favor something that leads to one's doom. This was a comment made on the Prophet ﷺ hadith that whatever befalls a believer is always good, and this is only for believers:
عَنْ صُهَيْبٍ قَالَ: قَالَ رَسُولُ اللَّهِ ﷺ: عَجَبًا لأَمْرِ الْمُؤْمِنِ إِنَّ أَمْرَهُ كُلَّهُ خَيْرٌ وَلَيْسَ ذَاكَ لأَحَدٍ إِلاَّ لِلْمُؤْمِنِ إِنْ أَصَابَتْهُ سَرَّاءُ شَكَرَ فَكَانَ خَيْرًا لَهُ وَإِنْ أَصَابَتْهُ ضَرَّاءُ صَبَرَ فَكَانَ خَيْرًا لَهُ
Suhaib reported that Allah's Messenger ﷺ said: Strange are the ways of a believer for there is good in every affair of his and this is not the case with anyone else except in the case of a believer for if he has an occasion to feel delight, he thanks (God), thus there is a good for him in it, and if he gets into trouble and shows resignation (and endures it patiently), there is a good for him in it.
— Sahih Muslim 55/82
So it is a matter of how we calibrate ourselves to receive said matters: a calamity may be to a reward in store that one could not have achieved otherwise through one's deeds, or a purification from one's sins in this world (which, by all means, is a much lighter punishment than on Judgment Day).
One has to realize that it is not one's decision to have a calamity befall one; whatever callamity that one is destined for will befall one anyway, and there is no escape. It is one's decision, not to question what is outside one's control, and to focus on what is within one's control: the determination by which one can face said calamities:
يَا بُنَيَّ أَقِمِ الصَّلَاةَ وَأْمُرْ بِالْمَعْرُوفِ وَانْهَ عَنِ الْمُنكَرِ وَاصْبِرْ عَلَىٰ مَا أَصَابَكَ ۖ إِنَّ ذَٰلِكَ مِنْ عَزْمِ الْأُمُورِ
O my son, establish prayer, enjoin what is right, forbid what is wrong, and be patient over what befalls you. Indeed, [all] that is of the matters [requiring] determination.
— Qur'an 31:17
Ibn Al-Qayyim in his book Zaad Al-Ma'aad,while commenting on Qur'an 2:155-157 said (paraphrased):
- If one examines thoroughly what has befallen one, one will soon realize that there are others who are in a similar or worse conditions. If one accepts one's calamity as a test, one will find in store greater rewards.
- The extent of the impact of the calamity can be contained, knowing that it could have been harder, and knowing that one still has many other blessings that one takes for granted.
- Panicking and questioning Allah' fate for one will not rid one of one's calamity.
- By not accepting one's fate, it does not go away, and what one loses is the reward for patience and submission to Allah's will.
- The reward that comes as a result of one's patience is beyond description. Allah summarized it as "give good tidings to the patient", and that a house of praise (Arabic: بيت الحمد) is built for patient ones in Jannah as a reward for his praising Allah and turning to Him.