I feel like a horrible girlfriend

I don't know why i can't be the best for her. It's really difficult. i want to be more accepting, nice, and sweet. I am all of those things but, i just can't feel like i'm doing enough. I am constantly judging myself saying "You're a deucebag or an ass." And i don't know if it's the truth or not. I don't really have anyone to tell me what's really going on. I have severe paranoia, i am on medicine for this, but i seem to still have this paranoia that if i mess up even slightly someone else will get her. Even though she thinks she might mess up, who the f**** else will get me. Seriously? I don't want anyone else but her. & yeah she thinks i probably want someone else. Well, just so you know Nicole. I only want you and only you.