My boyfriend's past hurts me too much...is there a way to deal with this pain?

You have to accept the past to move into the future. I have been in the same position, where I have been jealous of my boyfriends past girlfriends, but you just have to remember, he's with YOU now, and he loves YOU now, not her. He might have liked her before, but there isn't a spark anymore. So you should be happy and try to accept his past, because I'm sure if you had past boyfriends he would accept your past.