The OM told me he would be in touch, we'd have a coffee some time, but he hasn't contacted me. That makes me feel rejected and confused about what I meant to him.
So you can't move on because there was no closure. There was a promise that never happened so you feel lied to, neglected, rejected.
You know full well what you meant to him, exactly what he meant to you, fluff, excitement, forbidden. That was all.
What you have to realise is that you entered this knowing you "loved" your husband and was never going to leave him, it was just a bit of excitement, something missing in your life.
Hopefully, you've realised that what was missing you could have found without going to that extreme, that you knew that at the time, but you took the risk anyway, as you said the purpose was for "no-one" to find out.. Once you felt that "excitement" the penny would have dropped. "Oh, that is what I am missing, that heat, chemistry ".. At that point, you could have chosen to see what you could have done in your marriage to change that, but you chose to sleep with someone else instead.
In someways I have "some" sympathy, when a man continually persuades, lies, tells a woman she is beautiful when she feels that she is not, tells her she is awesome as a person, when she has been constantly put down. But, here, you are saying you had a good marriage, loving, great nothing like that at all, it just missed the spice.
You now have a skeleton in your closet and I hope therapy does help you understand that it is normal to feel chemistry, even lightly flirt but it is a braver person who stops right there, for love and then goes about working out how to re-create the missing link in her/his own relationship.
It's been 7 months but I get the "opinion" that you still don't have that excitement that you craved so much for that you choose to cheat to get it. I don't know if that is because of guilt? Although you state that the relationship is better as a result. But, if you were fullfilled, if you had learnt a very bad lesson, from this, then you would not be thinking of this guy 7 months later at all. If you had put all of your heart and soul into your husband through realisation and some guilt, you would not be thinking of this man at all.
Lastly, you are allowing your "ego" to get in the way. There was no closure, he promised. Who promised? Some guy you slept with ? Promises that are kept are kept from real people in your life that want you in their life and know that you are there for them as well.
This was a guy who flirted, held conversations and had dirty sex with you... Is he worth even remembering? Given you love your husband.
I say no.
If your husband is worth something, anything, then realise what that guy was and feel discusted both in yourself and also in him.. There is your closure, now work on yourself and your marriage.