I know exactly how you feel, I've been in the same situation. Like someone has already said I took the approach to never mention any previous guys I'd been with, which normally worked as he coped with my past by basically pretended it had never happened. This guy was pretty insecure which was the basis of his jealousy of my past, and it seems like your boyfriend may have the same insecurity issues. Just be really supportive and complimentary, although avoid direct comparisons i.e. you're much bigger/better than him!
But ultimately your boyfriend has to accept you for who you are. You are right, you can't change your past, and if he keeps bringing it up then you will have to tell him that there is nothing you can do to change what happened before you were with him. Your past has made you who you are today, and although he may not like it, he just has to accept it and realise that he loves you for the person you are now.