You're right to keep this one to yourself. Too many of us make the mistake of putting all our personal crazy stuff on our partner.
I can only give you my personal experience. Having been married, divorced and now with a partner of 20+ years, I can only reassure you that the things I did in my past had zero meaning to me when I started dating again.
Thing is, you might be wary that he shared engagement with her and not you. But if you marry him, this will be something he's only done with you. And marriage is bigger than engagement. And then you have a baby and a baby is bigger than marriage.
Each new thing you do with him will be something he hasn't done with anyone else. Heck, even if you ended up with a guy who's been married and had kids, making a new life with you is still new and exciting for him.
Think about it from the angle of sex. You've had sex before him, yes? Does this take the shine away from the sexual experiences you have with the new guy? Of course it doesn't. It's still wonderful.
If we can't reassure you, get some counselling. A counsellor can teach you how to deal with these panicky thoughts. You don't want to stuff up a good opportunity due to personal demons.
Last edited by basilandthyme; 02-04-15 at 05:17 PM.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.