It's hard to say, it depends on the situation, were you the one who broke up with your ex or was he the one who broke with you? If you broke up with him, you need to remember the reason why you broke up with him, which i assume you would have given a lot of thought , and you have to keep looking back at that reason. If he was the one who broke up with you, you need to be positive and say that he was the one broke up with you and that he is missing out on a wonderful person, so it's his loss. But in a time like this here is where you need your friends, go out and do things with them and keep your mind off of him. The more time you spend by yourself the more time you will have to think and the more you will miss him. The fact is that, he is going to move on with his life now and it's what you should be doing as well. If it was meant to be, what ever contributed to your slip up would not have mattered and you would still be together. What you have to do now is give yourself closure.