I agree with Lee. I think you should look into therapy for yourself. You need help learning how to deal with and change the negative thoughts. But, until you get into therapy, I would definitely recommend journaling. Put everything down on paper, get it out there, express it, but there's no need aim it at your boyfriend. The next day, you can go back and reread what your wrote, and point out the irrational fears/thoughts and negativity. Then rewrite these statements in a positive way... For instance:

"I think my boyfriend is going to cheat on me"

Changed to

"The idea of my boyfriend cheating on me is an irrational fear. He has never been untruthful to me, or give me any reason not to trust him. He loves me."

It's going to take time and a lot of practice. You might not believe what you're writing at first, but keep working at it. You have to retrain your brain away from the negative thoughts into positive ones.

Good luck! Keep us updated!