After 4 years, his exes are still brought up frequently.

After being with my boyfriend for 4 years (living together for 3) his ex-girlfriends and ex-wife keep popping up randomly in conversations. I guess I should've expected it early on from him, his friends, and family. Everyone likes to compare the new girlfriend to the last one. However, now it's annoying and actually starting to worry and depress me. I didn't even know he had been married briefly before (20 yrs ago) until a year after I moved in with him and his friend told me. Now he mentions her name, her problems, history, location, etc. He still had photos and posessions of the girl who moved out 2 years before I moved in. They were engaged briefly. He has a couple exes on his Facebook, tried to add his ex-wife but she declined... and his sisters and friends still keep in touch, so I avoid Facebook now. Sometimes I try to avoid conversations with his friends because they only talk about the past. I never bring up my ex-boyfriends' names. I've deleted them from my phone and social sites. My parents and friends don't mention them around us. I know half the problem is me worrying and being too sensitive. Lately I've been having nightmares about him marrying someone else.. and other times an ex shows up to our house and tells me: "I'm back. You can leave now." How can I help him move on and stop bringing them up and constantly reminding himself and me of his painful past... and THEIRS? He's bought a ring for me and I really want to figure out how to deal with this before we get engaged.