I've always observed that, for decent people who love each other, problems are almost never entirely one-sided.
I agree with Vince. She needs to get herself sorted out. I've read our brains don't really settle into 'adulthood' until around 25, so there may still be opportunity for her to mature (she needs to). Personality and response patterns get much more ingrained once we hit our 30s. If you are going to act, do it now.
As for you, again I agree with Vince. Sounds to me you've been a bit too indulgent of how she makes you feel when you are hurt or upset. How are your communication skills? You sound a titch conflict-avoidant to me. That behaviour comes with its own set of problems. I suggest you become more assertive--not aggressive--there is a big difference. Google - there are plenty of books about this. It will help you with interactions with her or whomever you end up with next. Good luck.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh