it was because things weren't working and they couldn't/wouldn't fix the problem. It was easier for them to walk than to fix. So an Ex is an Ex for a reason, don't stress.
Next, you need to realize that this person was an important part in his life and his memories of that time together, he has a right to keep and express, in fact if he didn't than i'd be worried (wondering what kind of heartless, emotionless animal your'e with). You can't ask him to erase years of his life because you get jealous and mad when he talks about them. They are just memories, it's not like he's replaying the most intimate sexual experience out load to you about his ex, it's just little cut-outs in time that come back when something reminds him of it. He doesn't do it on purpose, we can't control the things that come back to us, and when they come back, they just do. That's what memories are. Take any movie, somewhere in the middle take out some of the script. Now watch the movie with part of it missing. It makes no sense,right. Well that's what you are asking him to do with his life, remove just this part. Sorry doesn't work that way.