If he comes backs what do I do?

My ex broke up with me after a long, loving relationship. He was very mean to me the last two days of it and then broke it off with me for no reason. Many people say he will come back to me but i dont understand why? Whenever he finds out im talking to someone new he gets very jealous... I dont know why though if he doesn't claim he has feelings. I know that if he comes back I would want to go back but I know I will get hurt again... What should I do he does, tell him off?
If he comes backs what do I do?
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