You said something that is very important: that when you dated him, you were "ME" without thinking about being "ME." When one has been through the rejection of separation, it is hard to be oneself again if you did not build up confidence while he was gone that, dammit, it was not all your fault and no matter what you did wrong, you should have been extended clemency rather than be left.
You be YOU. Fix what needed fixing in the relationship, but don't beat yourself up or try overly hard. Marital love should be unconditional. You should not have to live in fear that you will be left again if you do not please him. It took a long, long time for me to get past that fear myself. And once you do, baby--you are liberated!
I am sad and stricken to hear that there was a relationship. I don't think my H was involved in anything, he says he was not, but that changes the picture for you. It means now that you have to live with the pain of infidelity--and it's infidelity whether or not he slept with her. And it does not matter if you were separated--separated is NOT divorced and does not give the partners freedom to date!!!
I can't stand to see your little heart be killed again. But here it is, and it is to be dealt with. In time, my guess is that you will want to know more but if he's not forthcoming right now, don't push too hard.
This does not mean that your separation was any "worse" than other separations or has less hope of repair. Not at all, and don't you think so. It does mean however that as a woman you have to deal with this revelation and it is a revelation that wounds grievously. IT HURTS.
I would advise you to talk with your counselor about it straightaway. I fear it will eat you alive if you do not.
Before my H left, I think he was playing footsie with a girl on the Net and he did go to meet her once while visiting a relative who lived near her. This was well before we separated and it was probably the final straw that made me not really care what happened even when he decided, that time, that he would stay. (Oh thank you for staying, Master, we only have a new baby you know!)
Later he said that she was really just a friend (I still didn't like that) and that he did regret even going to meet her. Good enough. Took me awhile to get over it, but I did. The really bothersome fact is that all the time he was silently building a case against me--all the time he was planning to leave--he was putting in time online with this woman rather than working on the marriage or looking as his role in our problems.
Lisa, in all honesty as your friend, I hear in your words somehow--and correct me if I am wrong--that you still maybe don't think you deserve your H. That you give him more status than you give yourself. You must never think that you are any luckier to have him back than he is to have you!!!! While it hurts to be separated and rejected, you must learn that your worth does not come from him. If he wiped you off his feet tomorrow like dirt, that is HIS loss. When you get that mastered, you will be less at the mercy of fearing he will leave again. HE DOES NOT DETERMINE YOUR WORTH.
Talk to a counselor. I can't say it enough. And if you are grieving very deeply, I say it again: take an anti-depressant for awhile (you don't have to tell your H). It does not make your problems go away, but it will make your mental and mood machinery function better so that you can cope. THIS REALLY SAVED ME AT MY LOWEST POINT--individual counseling with a counselor on my side (not our idiot marriage counselor who advised him to get an apartment) and medication. Trust me!!!
There's one other thing and it might not be much comfort but I hope it is: he did pick you. Over her. That he even sought her hurts, but he was confused and he made a common mistake. YOU, YOU, YOU. Now YOU have to believe in your worth, regardless of his actions.
Now wonder you have a hard time trusting. This makes it harder. Harder, but not impossible. When you begin to really believe in your worth and that you can cope with whatever he might do--because none of us have any guarantees about out mate, they might leave again or look at someone else--you will change in ways that communicates a new self-respect to him.
And Lisa, dear one, when you value yourself there is no way he cannot. (unless he is a total cad, and it's unlikely you would have married him if he was)