In my experience... You either find someone new (takes time), or stop believing that he was perfect (or the "one", or whatever else you want to label him or any other person in the future).
For me, I have not necessarily found someone new.. I thought I did for a bit (now I'm riding on hope, with patience). Stop believing that she was still "it" has helped me a lot in moving on. It is not by any means the whole solution to the difficulty, but it is a big chunk of it.
