My heart goes out to you. I feel your pain and the discouragement in your words. It seems like you are digging deep to find resolution to this situation.
My son is your age and one of his friends went through a similar situation. I spent a lot of time talking it through with him. So, I feel like I know where you are coming from to some degree.
It sounds like two things need to happen:
As for you. You need to forgive yourself. Right, wrong, or indifferent, you made your choice to be with an old girlfriend based on the information you were given at the time. You realized after the fact, it was a mistake for YOU based on your feelings for her. Notice, you didn't say you felt like you cheated on her. At that moment, you trusted her words in that, you were in the friend zone. It sounds like your guilt stems from betraying your own heart by sleeping with someone while you were in love with somewhere else. That guilt was later compounded by the fact that you now realize how much pain your mistake caused her, and the affect it has had on your marriage. You learned several valuable lessons. Don't be too hard on yourself. Remember, that one act does not define you or the man you are. The definition of the man you are comes after the mistake you made, which is: 1. how you dealt with it, 2. what you learned from it, and 3. how you apply it to the rest of your life. It is also important to note that when faced with the question, you owned it. You should be proud of yourself for your openness and honesty.
As for her. Her issues are rooted much deeper than that one act. Granted, what you did, didn't earn you any gold stars, but that is not the root problem. It is the problem that revealed the issues she has been suffering through. She needs your compassion, support, and help right now. Don't join her in the fights. Take a deep breath and try to de-escalate the situation, do not add to it.
My grandmother said true love doesn't die, but it does change and shifts as you get older. The hope is when you get to the other side of this, your love will be deeper.
My suggestion is to tie a knot and hang on a little longer. She needs to find a counselor to help her with her childhood abandonment and trust issues. Couples counseling would help too. Another suggestion would be to find an older couple that you both admire and trust. Maybe they can help mentor you as a couple. Sometimes we see happy couples that have been married 30+ years and think they have always been that way. Guaranteed they have been through some serous trials too that always broke their marriage.
Please remember she, nor your relationship, got this way overnight so, it will not be fixed over night either. Re-evaluate where you are in a few months.