First of all, do not, whatever else you do, chase him. Men are supposed to be the hunters. When you chase him, it actually makes you a lot less attractive. You need to seduce him with your feminine wiles which have nothing to do with the chase.
In fact, he needs to chase you. That's what men do. Men hunt, women choose. When you get this through your head, you will begin to understand how to get him back.
So, start by analyzing what brought you together in the first place. What was it that he saw in you that was so appealing? Do a harsh assessment of yourself. Have you gained a few pounds, done something stupid with your hair, or changed the way you dress? All of these things could have an impact on how he sees you.
When you say I want my ex back, you are bringing another person into your life. When he asked you out, you were a single person. But as you started dating, you became a couple. In the process, you probably gave up some hobbies, friends, or activities. In short, you changed. You stopped being the girl he initially dated. He may have become bored with the new you the you that you changed into for him. One way to get him back may be just to take up the old you so that you are appealing again. And then, don't change this time.
Another thing that could have happened was that he really liked you without knowing you. You were mysterious. But, as you became a couple, he found out things about you that he didní't like. Perhaps he found out that you swear, and that goes against what he thinks is appropriate for his girlfriend. Think about the things that he harped on you about. If you are really saying I want my ex back, consider making the changes he asked for.
Once you have all of the pieces in place you are the girl he really wants then you have to seduce him.