I recently went through a difficult divorce, so I can relate to some of what you are saying. No matter how awful a marriage is, sometimes we see things differently when we are grieving, and our judgment is clouded by our emotions and all the 'what ifs'......especially if the other person seems to have moved past the breakup. The important thing for you to remember is this: you left because you realized that nothing was changing in your marriage and that nothing would.....you said so in your post, and I know that you know this to be true. Grief does strange things to our thought processes and our emotions, and it's natural, I think, to second guess our choices and wonder what would happen if we could go back. You made this choice for a very good reason, and you need to remind yourself of that. In my own case, I spent time writing down all the reasons why I left, and purposely wrote down memories and examples of those reasons, to help me to stay strong in my convictions. I was firm in my belief that I did the right thing, and anytime that grief took over, I took out my journal, and looked at what I had written, and gave myself an important reality check on what REALLY happened during the marriage. You might find this helpful, too.
I also went to see a therapist when I left my husband, which I found to be very beneficial, particularly since she was able to ask me those 'reality check' questions and give me an objective assessment of the situation and the feelings I was having. Perhaps this is something you would consider doing as well.
Please come back and post to us again as you feel able to. We're here to support you, and to listen to what you're going through.