MairI, I feel for you.
I believe you are very young, and it is a fact your brain will keep developing until you are 25. As will your boyfriends. That is why I believe you made youthful mistake and you have learned from that mistake. I don't put you in the same leauge as people like myself, 52 years old, who knew better.
From reading your past posts, I don't think your boyfriend has matured to a point that you have yet. Really, threesomes after saying how much he doesn't trust you, yada yada. Him not being open, and from your accounts he never has been. He is no more able to have a comitted relationship than you have to this point. I still don't think you got the truth about his ex or the naked girl in the bed, sorry.
Cut your losses.move forward with your life and don't make the same mistakes again. There will be many many opportunities for you in the romance department. You made a mistake and you learned from it. You have put in the effort to change it and it has gained you perspective. We all learn in that manner. Some may have had the ability to not make mistakes and learn, I didn't however. We are all very similar we humans.
Don't keep beating yourself up about this. You are a human being who is going through the process of life and you are doing well learning your lessons. See who else is out there, I'm sure you will be amazed at the possibilities. Leave your boyfriend to make his own mistakes and find his own way.
The fact you are on this board, asking questions, sharing info on yourself, doing the self searching is evidence that you are not a cheater and a scoundrel. Give yourself a break.
If you are religious (I am not by the way) and you need absolution and will accept it from a mere human
then you are absolved. You made a young mistake (yep, I stand by that) and you don't need to twist yourself into knots over it and you need to enjoy your life with your new found knowledge about life and love. You never made a marriage comittment, which of course puts a slight different spin on things, but not in your case. I believe your boyfriends immaturity will ruin the gains in self knowledge that you have made. Run.
Yes my last bit was tongue in cheek, hope I didn't offend anyone, but it seems you are needing to be told it is okay, you are okay, you can move on and feel fine about it. You can.