Do not seek to blame his former partner, most often to be found the cause in itself. But to engage in self-flagellation is not worth it. It will not do any good. Find the middle ground. In any conflict situation, which can lead to separation, blame is usually both partners. Therefore, you must first analyze the situation, to understand what was wrong, and begin active work on his character.
In the next step you need to cure himself torn away from this painful history. To do this, the whole situation can be described on paper, nothing embellishing and not underestimating. Describe the pain that you become as a result of torture. Then you need to break this note or that will be even better to burn. So you like to erase your pain. Then take another sheet of paper on which describe what you would like to learn what qualities are not enough to make such a situation you do not cause acute pain.
Now it's just up to you daily and hourly work on yourself and your character. Sheet save. Every time you manage to learn something, mark it on your list.
Now you need to escape. To do this, try as little as possible to be alone. Visit crowded places, communicate with friends. Sign up for any courses or go to university. Then the time for soul-searching you will not have, treatment will be more effective.
And most importantly: do not dwell on his unhappy, and now unrequited love. In life there are many things that are more important than predation. Get a kitten or a puppy. You will be something to do, someone to take care of, and then cure the pangs of separation will occur faster.