Feeling insecure

I'm 27 have been in a relationship for almost 2 years and still feel so insecure. My significant other used to talk a lot about his exes and even though he doesn't anymore it still eats away at me as though they're going to just walk back into his life. I've been cheated on he's been cheated on I've done things I regret in a previous relationship I just want to know how to put all of this in the past and stop feeling guilty for things that have nothing to do with this relationship. I hide it all but that's not fair to him or me. Please any advice will be much appreciated.