Side chick

I'm currently the side chick to my high school ex. I'm 27 now dated him when I was 16 only for a couple months. We've been in each other's lives for what seems like forever at this point.

He's been dating his girlfriend for about two years. I know her cause she went to school with us for a while.

Him and I have great conversations and we open up to each other more than with we do with other people. Obviously, there's a sexual attraction as well but we've never slept together.

We're both happy with each other and the situation and I don't want a relationship with him because if he so easily can cheat on his girlfriend then what stops him from doing the same to me.

We're both willing participants but I'm scared that this could go on for a long time cause neither party wants to end it.

Fun twist: we haven't physically seen each other in 2 years.

Do I just keep it up and be happy with how everything is or do I cut it off and actually try and move on?