Why does a lie from 9 years ago still bother me?

I knew his ex from school, and he seemed to have a lot to say about her early in our relationship even though they were only sort of a couple for like two weeks. She was a bad person and treated him awfully and he broke up with her. I told him I would appreciate it if he didn’t contact her or have anything to do with her and he asked the same of me about my exes. It wasn’t a problem for me and I thought it wasn’t a problem for him. We moved in together and after a year or maybe 2 years I got on the computer that we shared. He left a Facebook page open that was not an account I knew about. It seemed like it was incognito. He had the page open to messages sent between him and his ex that I was clear I didn’t want in our life. She was the only friend on the account and the only conversation. Nothing seemed like he was contacting her to cheat, there really wasn’t much in it. When I confronted him about it he denied it and acted like he didn’t know what it was. Later he deleted his history and I was unable to find the page again. After fighting for weeks, we made up and he still insisted he didn’t do it. I know he did, I just chose to let go. I thought about it from time to time but especially this year it has been eating away at me. I don’t want to bring up the past and dwell but I almost feel like I can’t move on until he admits it. Like it would be closure for me and I can trust he will tell the truth no matter how embarrassing it is. We just had our 11 year anniversary so I knew he’s in it with me and me only, but I am still tortured. Why does this bother me so much still?