&gt;I really care about her and felt like I was actually going to be happy for once.

Don't rely on other people for your happiness. If you are unhappy in your own life or with yourself, spend some time getting introspective and identifying why. Once you know the why's the question becomes how to change that. You should be a whole, complete, happy person even when you're single. Getting into a relationship should just be adding the right person into your already complete life. A preference, but not a necessity in order for you to be happy. That will help you a lot with maintaining a healthy relationship.

As for the girl...

&gt;She told me she had slept with one of her exes just before we started talking (as in two days prior). Everything she told me about this guy is terrible and she talks about how much of a piece of shit he is; how he would manipulate her, control her, etc. Which is why I still can't comprehend why she would try and get back together with him...

This happens all the damn time. Abuse victims, if that's what she was, go back to their abusers a depressing amount of the time. Cheaters tell you about how someone is like a brother to them, or disgusting, or how they hate them, or how they could never have feelings for them. People who aren't over their exes can say all kinds of mean things about them and then still end up getting pulled back in. Don't listen to her words here. Watch her actions. 

Whether she keeps the pregnancy or not, discussing it with her ex is not a good sign. Right or wrong, he doesn't really get a say over whether she keeps the pregnancy or not. So she wouldn't be discussing it with him unless she was considering keeping it... and even then she wouldn't need to discuss it with him unless she was interested in some form of relationship, even if that was just co-parenting.

&gt;I guess I have to end it?

That seems likely at this point, but you don't have to jump the gun. Let her know that if she wants to go through with the abortion and cut off her ex you'll be there to help her through it as much as she wants you to be. But also ask her to let you know if she's planning on keeping it or remaining in contact with her ex. She doesn't necessarily need to make a snap decision on that, but she needs to make a call fairly quickly on that front. I'd tie maintaining contact with her ex to going through with the pregnancy, those both being a deal breaker for you. Abortions are fairly emotional, and there's an emotional tie to him surrounding it. If she indulges in it then you should probably cut your losses. It will very likely end up hurting worse if you hang on at that point.