Wow Lovely, stay busy! Concentrate on your life and maybe making a few changes so it does not feel "mundane" Do whatever you need to do to get this man out of your mind. It sounds like you have a marriage that many people would give anything for. You are right the grass usually does look greener,and you have not been with this other man for 20 years. You broke up for a reason . Think about how you would feel if your husband confided in you that he was talking to an ex!
If sparks are not flying in your marriage, and you want them, make them happen. Plan romantic dinners, seduce him, make it hot again.
Dont waste your time on what if's when you have such a wonderful man.