Should I leave him?

My husband and I have been married almost 3 years.

Before we were married when we were dating, I found messages between him and a girl. They met through tumblr and lived in different states so they never actually met but still exchanged messages online. I had confronted him about it and we worked through it and got married two years later.

Last week, my sister found messages on her husbands phone which prompted a discussion between my husband and I about what happened.

He now admits to more than messages which made me livid (I’m not necessarily heartbroken since I knew that was always a possibility but he has denied it for almost 5 years). Hearing him admit it somehow just made me relive it all again and now knowing for sure he did more than just message makes me think I can never trust him again. He’s adamant that he never had feelings for her and has never done anything like that since and it was a one time mistake.

I just don’t know if I should believe him all over again or leave. For reference, we don’t have kids but we do own a house.

What should I do? It’s apparent I lied to myself about “moving past it” the first time since I still bring it up in really big arguments we have but I also didn’t bring it up all the time and haven’t gone through his phone because I did trust him again. In fact, I’d forget it even happened for months at a time but then something would trigger my memory and it’d anger me all over again.

I know I shouldn’t have married him if I was never truly past it but that ship sailed almost 3 years ago.

Any feedback is appreciated.

Thank you for listening.