All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
We hope that what happened to you with this girl would be a lesson for you and for all those who take sinful relationships lightly. We have already issued fatwa 81356 warning against this.
You must hasten to repent immediately, whether in regard to having this relationship or in regard to helping that girl in aborting and killing the fetus. Your repentance is not dependent on marrying her, and you are not Islamically obliged to marry her; rather, it is not permissible for you to marry her unless she sincerely repents. For information on the conditions of repentance, please refer to fatwa 86527.
If she sincerely repents to Allaah, then you may marry her, and supplicate Allaah as much as possible as this is one of the most important means for achieving what is required as Allaah is near and answers the supplications. Allaah says (what means): {And when My servants ask you [O Muhammad] concerning Me - indeed I am near. I respond to the invocation of the supplicant when he calls upon Me. So let them respond to Me [by obedience] and believe in Me that they may be [rightly] guided.} [Quran 2:186]
For more benefit, please refer to fatwa 188393.
If marriage is facilitated with her, then all is well; otherwise you should look for another woman to marry. As for love sickness, it can be treated; to know how, please refer to fatwa 84544.
You should not grieve for not marrying her but instead ask Allaah to facilitate for you someone better than her, as you do not know what is better. Allaah says (what means): {But perhaps you hate a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allaah knows, while you know not.} [Quran 2:216]
Finally, we advise you to refer to fataawa 85970 and 98739 on the rulings of abortion.
Allaah knows best.