This is such a difficult time for you, I am sending you hugs and one suggestion.
I picked ONE thing I did that was a symptom of my hanging on (e.g., checking my email, accepting a date with him, or driving past a place where I knew he would park his car) and told myself that I could not do that one thing, just that one thing. Even if it meant changing my schedule or not checking my email for a certain number of days or pretending I had plans for the evening. Doing this one thing gave me a little space, and soon the space got a little larger, and I could breathe again.