Ok, it's gonna hurt like hell for a while. They say it's a bit like a bereavement. You've had this image in your head of a new romance, a possible new father figure for your kids, a relationship that has so much promise. You've probably made plans and had so many fantasies in your head of the kind of life you would lead together. Losing that is profound. Take time to grieve for that fantasy life that you have lost. It would definitely help if you had no contact with him, although that won't be easy.
We can all say that you will find someone else, that it is better to learn these things now rather than later, but it is of little comfort to you right now. Answer me this though, do you think you needed him to be happy? Do you not think it possible to be happy without him? When you were apart, were all your thoughts focused on him and what you would do when you were together again? What I'm suggesting is that maybe you were in love with the idea of him, rather than the man himself. That he failed to live up to your expectations is a huge blow to you. What you need to do now is to focus on yourself for a while. List all the positives in your life, such as your children, etc, to remind yourself of what you do have. Ask your friends for any positive comments on your character. Remind yourself of what a wonderful person you are. You might not feel much like talking to anyone or going anywhere, but believe me, once you have made that effort you will find that it is better than you anticipated.
You've made the first step. You've already acknowledged to yourself that it is over. Allow yourself to feel sorrow, anger, resentment, etc. Just don't let those emotions swallow you up. Call old friends to chat to, treat your children to the cinema or something. Get a book so that when you are by yourself you have something to focus on. You will start to feel better, it'll be a tough few days but you will come out of this a better, stronger person.