I'm feeling really paranoid that my gf is having an affair with a coworker, i'm always looking for clues and i don't like that i am doing this at all?

Nice that YOU say it: I'm so impressed that it bothers you. Usually we hear complaints from the other. It's normal, but you are right - it's not good - to feel this way. It may even be built into our species. We can do like you and try to surmount it. A counselor (as wise as you or better) may help. I once knew a psychiatrist who recommended brief meds to blot out sexual jealousy in otherwise normal folk. Talk to her.