I feel your pain. Getting back to building your own life, when you thought you were building a future with someone can be daunting at first.
It helped me to throw myself into exercise, long drives with great music, to towns I hadn't yet explored. It took time and some therapy as I left an abusive ex husband but happiness is a choice and grief is a process, as the saying somewhat goes. You deserve to be happy.
I can only speak from personal experience and that of close, personal friends, but a guy who can carry on a long term affair like that doesn't seem to usually change. They get away with whatever they can as long as they are able and become so skillful at lying along the way, how can we know when they are being genuine?
Maybe trying different teas would help with the afternoon slump? Writing a letter to him that you DON'T send helped me, too.