I am currently 18 years old and I have been thinking of converting to Islam. However I am haunted by the fact that I recently had an abortion at 6 weeks. My boyfriend and I are getting married soon. I would like to know how we could go about asking God for forgiveness. I truly regret my decision and cry almost daily and I know he feels terrible as well. I would like to know if we can still be forgiven even after such a terrible sin. I have been reading about what is haram and I am aware that I have a commited a sin. Please help me. I do not want God to be angry with me.
First: Abortion is both a very personal decision and a medical matter. Feeling conflicted about it simply means you're a decent human being.
See Islam and Abortion, Wikipedia for more about Islam's perspective. Abortion is considered forbidden after the 120 day mark:
... every one of you is collected in the womb of his mother for the first forty days, and then he becomes a clot for another forty days, and then a piece of flesh for another forty days. Then Allah sends an angel to write four items: He writes his deeds, time of his death, means of his livelihood, and whether he will be wretched or blessed (in religion). Then the soul is breathed into his body. -- sunnah.com
As for before the 120 day mark, it's more complicated:
Some sages or scholars, however, were of the opinion that abortion is permissible up to a certain point—some arguing that abortion is permissible up to 40 or 42 days (this is the position of some Shafi’i jurists) or up to 120 days (the position of some Shafi’i and Hanafi jurists). But most of the jurists who argued for the permissibility of abortion (whether under the 40, 42, or 120 day rule) also contended that unless justified by a health necessity, abortions are reprehensible or disfavored (makruh). -- Dr. Khaled Abou El Fadl
Second: Generally in Islam, one is excused if they commit a sin out of ignorance:
Whoever commits a sin while not knowing that it is a sin, he will not be accountable for it, as there are many evidences which prove that a person who does a sin unknowingly is forgiven. -- IslamWeb, on zina
Whoever does not do an obligatory action because he does not know that it is obligatory, or he does a forbidden action because he does not know that it is forbidden, is an ignorant person who may be excused for his obedience. -- Islam Q&A
Third: Allah forgives all sins from the truly repentant.
Say, "O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful." And return [in repentance] to your Lord and submit to Him before the punishment comes upon you; then you will not be helped. -- Qur'an 39:53-54
Fourth: It's widely accepted in Islam that those who convert to Islam have their prior sins erased.
Say to those who have disbelieved [that] if they cease, what has previously occurred will be forgiven for them. But if they return [to hostility] - then the precedent of the former [rebellious] peoples has already taken place. -- Qur'an 8:38
He (the Holy Prophet) observed: Are you not aware of the fact that Islam wipes out all the previous (misdeeds)? -- narrated on the authority of Ibn Shamasa Mahri (sunnah.com)
It's discouraged to openly discuss pre-conversion sins, however.
Allah is full of mercy and kindness and is always ready to forgive. Never despair of His mercy. Allah says:
قلْ يَا عِبَادِيَ الَّذِينَ أَسْرَفُوا عَلَى أَنفُسِهِمْ لَا تَقْنَطُوا مِن رَّحْمَةِ اللَّهِ إِنَّ اللَّهَ يَغْفِرُ الذُّنُوبَ جَمِيعًا إِنَّهُ هُوَ الْغَفُورُ الرَّحِيمُ
“Say (on My behalf), ‘O servants of Mine who have acted recklessly against their own selves, do not despair of Allah’s mercy. Surely, Allah will forgive all sins. Surely, He is the One who is the Most-Forgiving, the Very-Merciful.” (Al-Zumar 53)
Alhmadulillāh, Allah has put in your heart the desire to change and become His obedient slave. This is the first step towards tawbah. Capitalise on this feeling and do not give up or become despondent.
First of all you should know that one of Allah's or God's names -in Islam- is the all forgiving or most merciful!
Allah, our Creator, exactly know that we are human and that we are not infallible (read for example verses like 4:28, 17:83, 39:49, 41:49 42:27 and 89:15-16), and he is caring for us more than a mother as quoted in a hadith. Therefore he knows the malady and provides a remedy: repentance. So if you do sincere repentance from what you did Allah may forgive you.
How Allah expect us to be
In the last part of surat al-Furqan (25:63-77) Allah even discribes those people whom he calls the servants of the most merciful, showing off how they should be or act and how they shouldn't be or act (so these verses are of the kind: do and do not)... I strongly suggest you to read and re-read this part as it is a gudiline for us in our interaction with our Creator. In this description you can read in (25:68)
And those who do not invoke with Allah another deity or kill the soul which Allah has forbidden [to be killed], except by right, and do not commit unlawful sexual intercourse. And whoever should do that will meet a penalty.
so these people should not kill a soul Allah has forbidden to kill, and if you go to the next verse you may read about their punishment (in case that they went ahead doing these bad deeds) before comming back to those people he called the servants of the most merciful, the people i might call the people who have been guided by Allah to a good or correct behaviour in their life, and quoting them as an exception:
Except for those who repent, believe and do righteous work. For them Allah will replace their evil deeds with good. And ever is Allah Forgiving and Merciful. (25:70)
so if one repents Allah sincerly Allah my turn his bad deeds into good deeds (for example, by guiding him do good deeds and turning him away from bad deeds).
Then the verses quote what happens if you repent:
And he who repents and does righteousness does indeed turn to Allah with [accepted] repentance (25:71)
and later adding rewards to these kinds of people:
Those will be awarded the Chamber for what they patiently endured, and they will be received therein with greetings and [words of] peace. (25:75) Abiding eternally therein. Good is the settlement and residence. (25:76)
Allah accepts sincer repentance
About the mercy of Allah and his acceptance of repantance you can read many other verses (most of them have already been quoted therefore I just added the links) such us:
Surat at-Tawba (9:104) I would even suggest you to read more context like (9:102-105)
Surat az-Zummar (39:53)
Surat ash-Shura (42:25)
Verses 1 and 3 just tell as that Allah accept repentance (if it is sincere as explained above) while Verse 2 tells us don't worry, be faithfull and believe in Allah's mercy and forgivness!
Further in surat al-Baqara (2:222) you may read:
... Indeed, Allah loves those who are constantly repentant and loves those who purify themselves.
and also in surat Taha (20:82) you find:
But indeed, I am the Perpetual Forgiver of whoever repents and believes and does righteousness and then continues in guidance.
this means if you repent, if you left all the sins you committed Allah even would love you this means you shouldn't have any haram relationship, but look for halal and do good deed, this means you should live or start a new life forgetting all the past (bad deeds) and leaving it behind.
A "Bonus" for those who convert
As you said you consider converting I wouldn't quote what Islam says about former committed sins nor the scholars view on the sin that you feel terrible about, but I'd say Allah offers you a solution that might clear your past and let you live with a new white register of deeds (8:38) which is more explictly quoted in a long hadith from which I'll emphasize the statement:
Are you not aware of the fact that Islam wipes out all the previous (misdeeds)?
So repent, leave the past and any bad deed you committed and find your way to God and he might forgive you and even love and reward you!
Maybe this fatwa is also helpful.
God is within you and everywhere around you. God knows what happened and God knows how willing you are to be forgiven.
He is the One who accepts the repentance from His servants, and He forgives the sins. He is fully aware of what you do. 42:25
Keep the contact-prayer with God, He is always listening, so speak your mind :
When My servants ask thee concerning Me, I am indeed close (to them): I listen to the prayer of every suppliant when he calleth on Me: Let them also, with a will, Listen to My call, and believe in Me: That they may walk in the right way. 2:186