Your last sentence says it all. EVERYTHING WILL BE OK.
You mentioned that your girlfriend is 21. How old are you?
The main question to ask yourself is........ Do I love this person? You must be honest. If the answer is yes, then I think you need to pursue the relationship. Yet, 21 is a very young age and unless both of you are mature enough to continue a serious relationship, I would hold off on marriage. Make sure you are right for each other and make sure that the love you feel for each other is real. Time will withstand the test.
It looks like she has been through a lot. And a lot of pain. By telling you about her past, openly and candidly, she took you into her trust. I don't believe this was easy to do. About her "past" bothering you, and as I say this keep in mind that I don't know her and I don't know you. Yet, she took you into her confidence and shared her heart openly. Maybe she loves you this much? So... where are you now? Are you willing to start from this moment on and look to the future with the girl in your arms NOW, or are you going to go back to the past and fester on what "was." The past cannot be fixed. But the future can be bright and beautiful with the one you love. The sex part is not clear to me. Have you both agreed not to have sex until you are married? Did only one of you agree on this? Or........ Did her abortion cause such pain that she is careful, hesitant, what were the circumstances, are there trust issues?
From the girls I have come across, EVERY ONE OF THEM, except for one case, abortion is not an easy thing. It leaves its emotional scars to be carried for the rest of one's life. I have found, except for one instance, that the abortion was undergone because of panic in one form or another. The circumstances for this panic are many but a pregnant girl can be very afraid and alone. The saddest of all is that there is no closure once the abortion is done and over with. An abortion is a very serious physical and emotional undertaking. And that, also can never be undone. Chances are she will carry that with her for the rest of her life. Yet, it is very important that she somehow heal from this.
Please realize that she has taken you into her trust and she risked an awful lot by doing this. Know that she did not have to tell you all this.
So...... If the answer is "yes" to that question..... "Do I love her?" Put the past behind you and help her to put the past behind also. Be there for her. Look to the future that you can possibly build together. But go slow. Make sure its right. And if it is true love, it will only grow.
I hope I have helped. And hey........ let me know how it goes.