Dear SueDev
You seem to have identified a key part of the issue that you are facing and that is forgiving yourself. I wonder are you a perfectionist? I think most maried people if they sat down and were completely honest would say that they had hurt their spouse at some point. While heading down the road towards infidelity, you did turn back and sort things out with your husband. You cannot turn back the clock but you can move forward together.
I don't know how best to advise you but I will share something that happened to me. I am a Christian and so my experience may or may not ring bells for you, but here goes.
One day I was involved in an altercation with someone, and when I came home I suddenly realised how unkind and thoughtless I had been. I was devastated and desperately wanted to go and find the person concerned and apologise. Unfortunately she was a stranger and I didn't know where to find her. I felt ashamed, embarrassed and distracted. I started praying about it and I felt that God was saying to me, that I wasn't actually interested in the other person's feelings, but I wanted to put things right so this person wouldn't see me in a bad light. In this case for me pride was involved. Part of the Christian faith is that we can't get things right all the time which is why Jesus came to pay the price for our sins. Otherwise we would be carrying the guilt of many of them around. I was able to move on in the end by recognising that I had hurt this person and there was actually nothing I could do to make myself look better again and to heal the hurt. I could not change the situation, but I was still loved by God.
I don't want to draw too close parallels with your situation, but you have been able to seek forgiveness and healing from your husband. He has forgiven you and loves you despite this, just as you love him despite the pressures from his family. That sort of love, accepting us warts and all, is the sort of love which sustains marriages. You and your husband have something very precious. He wants you around, with him as his soulmate for life. Accept his love, whether you think you deserve it or not. Few of us do really "deserve" love - it is a surprising and wonderful gift.
If I might suggest that you could also consider what
forgiving and accepting your mother in law might look like. I suspect that she isn't going to change. She is trying to hang onto the past relationship with your husband as her little boy and that is not healthy. My children are just leaving home and forming adult relationships and I have struggled with jealousy that others are now more important to them. I can understand why your mother in law is behaving this way but I don't condone it. There is an interesting resource
here about in-laws. Do read it and see if it gives any clues as to how to respond to your mother in law.
You can find wholeness again. It may take time, but your family needs you and they love you.
Liz