Ah, crap it's happening again, what do I do?

Guess what? YOU STILL LIKE HER, and it's okay! That stomach-turning, roller coaster feeling you described is likely how your body manifests anxiety and each time you hear about this girl or see her, that feeling comes back. Sounds like you have a choice to make: either try to get back together with her and see if you two can make it work this time (you'll probably have to tell her exactly what's been happening with you if you ever want it to stop happening!) or get on with your life without this person. Tough decision, I know, but if you take some time and really think about what you want, you'll no doubt make the right decision. Good luck!