How could my ex-boyfriend leave me in such a difficult situation?

I'm sorry about what you went through...a very traumatic experience. You said that you both have had some ups and downs and this probably shocked him into making a final decision to end things. He could also feel some guilt about what happened to you...and almost becoming a father when he wasn't ready to. After all, if he didn't have with you, there wouldn't be a pregnancy or eventual miscarriage. If he doesn't want to stay, you can't make him. All you can do is find some way within yourself to let go of him and move on with your life. Could be spending time away, hanging out with friends and family, counselling, doing more things at school...whatever keeps you busy and will help you feel better. If he runs off now when things are rough, you probably won't expect much better from him as a husband or father. Keeping your distance and working on helping yourself will do more for you than plotting revenge. Let him go.