I imagine many many men have been in your situation before. You seem to have a good understanding of how completely horrible this was for her. She thought you and her had a future, hence the sex. She wanted the baby, but knowing you didn't, aborted it. She will never forget this and never forgive herself for it. I know you feel bad and I get the impression that maybe this episode has made you grow up from a boy to a man.
My suggestion is that in future you understand this: sex is called lovemaking for a reason. Women read far more into a kiss, let alone sex, than men do. For her it was a big deal. Can you blame her for assuming you were into her?! Why not wait until you are in love before you next have sex. This way it will mean something and won't be the end of the world if pregnancy does occur. Women tend not to treat sex lightly as men do- understand this. And needless to say, if you don't want to be a Dad yet, use a condom.
You have acted like an arse, but it is refreshing that you actually realise this- so many men don't. Put it down to experience. Try and be a good friend to her. Explain to her how you feel- like you have done on here. And don't do it again!!