My gf doesn't trust me anymore she always get paranoid...?

my gf is worried that I'm cheating on her but I really explain to her that I made some mistake in the past. I never cheat on her but she always keeps somehow argue about it we we have a little fights. We been together for two years and she is going back to school and another year of long distance. How do i make this work out? I know she isnt going to trust me becuase i have to work hard for it, thats what she always tells me. So another year of heartache and pain.