Since I started nannying I've been a long-time lurker on Mumsnet. However, I'm a first-time poster. I made an account just to share this quote with you, OP:
"If you are to be a positive influence on the world, you need to forgive yourself."
Ask yourself this - who benefits from you beating yourself up over this? It doesn't do your child or partner any good for you to devote your energy to criticising yourself over mistakes you made at eighteen. It doesn't do the wife of this man any good either. It does nobody any good, least of all you. Like another poster said - if you had a daughter involved in a situation like this, how would you speak to her about it? Wouldn't you tell her to be kind to herself? Be kind to yourself.
Also, ignore the above comments. It's pretty telling that the word "whore" has actually come up in this thread. People tend to do a lot of projecting when this topic comes up. It's a very painful subject for a lot of people - try not to take it personally - a lot of people have chips on their shoulders which obscure their vision when it comes to OW, and while that's completely understandable, you don't deserve to be a scapegoat for the actions of any man.
If a married man leans in to kiss another woman, and she pushes him away and says "No, you're married" - HE HAS ALREADY BEEN UNFAITHFUL TO HIS WIFE. It is not the responsibility of every woman in the world to ensure a husband stays faithful by ensuring he never gets an opportunity to cheat. It is HIS responsibility not to cheat.
My partner once kissed another woman, just for a moment. She leaned in to kiss him; he let her. He told me, I was angry but I decided to let it go. It would never have occurred to me, even for a moment, to blame her or be angry at her. She doesn't know me, or owe me anything. My partner is the one who has made a promise to me. My partner and only my partner is responsible for his own actions.
You were eighteen years old and had no concept of what life was like for his wife at home with her small children. It makes total sense that now you understand that, you feel guilt. But you don't deserve to feel all this guilt. In my humble opinion, you weren't really the one in the wrong. It is wrong for a spouse to cheat on their partner. It's not great for a grown man to sleep with an inexperienced, insecure teenager at all. I'm in my twenties and would politely turn down any sexual offers made by 18-year-old (even if I was single) because 18 really is very, very young. In all honesty, fuck this guy for making your first sexual experience something you would look back on with such distress ten years on. It's a shame you didn't meet someone closer to your own age first and have a more natural experience.
But what's done is done, and can't be undone. So forgive yourself. Be kind to yourself. Get therapy over your traumatic birth if you think that might be the real underlying issue. But most of all, be kind to yourself. And DO NOT BLAME YOURSELF FOR THE ACTIONS OF OTHER PEOPLE. You did not make him cheat on his wife. Nobody can make a man cheat on his wife. Men are sentient beings capable of independent thought, would you believe it, not inanimate objects which can be hidden in one's pocket and stolen.
I know this is a very long post, but you've received a lot of judgement and nastiness here which you don't deserve.
If one of my girlfriends were telling me this story, she would receive nothing but compassion, empathy and kindness from me, and I hope some of that reaches you across our keyboards and screens.
Now its time to show compassion, empathy and kindness to yourself.