Well if he keeps telling you new stuff about other girls, why don't you just ask not to talk about what went on in the past? Honesty to the point of spilling your guts about irrelevant things is definitely not the best policy in this case. My boyfriend's 9 years older than me and, by extension, obviously has more experience than I do. But he doesn't talk about any past relationships unless I ask, and hard as it is to think he's comparing me with them, I just have to trust that he's with me for a reason... because I'm better! And so are you, otherwise he wouldn't still be there.
You should really talk to him about it though, if it's bothering you this much, I'm sure he'll reassure you that he's with you and not them for a reason. Good luck sweetie, I know it's hard but it's so worth it if it works out.