What should i do in my relationship?

what should i do? i have been dating my gf for a year and 3 months now. she went away to college in orlando about 8 months, but i live in miami so we get to see each other every weekend. we spend much time together and i thought i loved her. i even gave her a promise ring this past christmas. im 18 and this is my first real relationship, but its not her first. i lost it to her and maybe that has made me more attached. i dont feel happy with her and i guess im just too overprotective. i dont trust in her as she messed up once during our relationship, though it was more than a year ago. i cant seem to ever find myself at peace and i constantly feel she is doing something wrong though she has made a complete change and renewed herself into a new girl since the incident. i graduate in barely 3 months and i am planning on moving up there also, but now i am reconsidering whether i love her or not. deep down in my heart i dont believe in her though she has shown me that i should. what do i do