First, you must create a space before another can fill it. As long as you are living in the past with what might have been, there is no room for another. Time is the only cure.
And, while you are at it, examine yourself, and ask, "What is MY role in this"? As long as you insist on laying ALL the blame on him/her you will be stuck in the past.
We have all been victims of our own poor choices in mates, and the problem is not "Out there" anymore than the solution is "Out there"

I could easily say that my ex was a pain in the ass and impossible to live with. That is such a comfort to be able to forgive myself while laying all the blame on her, but the truth is, "I was unable to adjust to her way of thinking." See the difference?