I had that problem with my boyfriend. My past was constantly getting in the way. I was lucky enough to have a boyfriend who was very understanding, and has helped me a great deal. It has to be worth telling him hun. It will be hard at first, but you will feel better for telling him. He will be understanding when you react badly to something that he does... he'll understand. And from my experience, it puts you at ease in your relationship.
If you find speaking to him hard, you could always write him a letter to explain things. I interchanged between talking and writing, and it was really useful. Good luck hun.