Confused

My girlfriend left her boyfriend for me and this past year has been pretty emotional and difficult for both of us. We've faced a lot of obstacles and I want to believe that we've overcome them and are in a better place but I just don't feel like it. I don't know what to do, I know that she still loves him and I just feel like second best. Part of me wants to be single and part of me loves her, I'm so confused I don't know what to do. In the beginning I was happy but for the past few months I've been feeling down and I don't know if I see a future with her because she's broken my trust so many times, by lying to me about doing drugs multiple times and kissing her friend and I am just paranoid all the time. I'll take any advice I really need it.