I need help regaining trust in my girlfriend and earning her trust back?

I know it's hard but I think you've just got to stick it out and trust her. she obviously loves you because she's giving you another chance when she's already been hurt so so you must be good enough. If you don't think you can do all that straight away then talk to her, tell her why you feel insecure and ask her to help you too by not flirting :)