Continue to get a life and have fun for yourself. Don;t just do it to get him back. If you are happy and moving on with your life, it will confuse him.
If you want to, hire a Private Investigator. Get evidence of the affair. It will set your mind at ease (if there is no OW), or at least, let you know what you are up against.
It's OK to let him know you know.
But then, nothing else changes -- you start living a fun life.
