If you did something to make you guys break up or whatever, you might have to earn his/her trust back. But sometimes that doesn't even work. It depends.
If he/she broke up with you, and you were totally attached, don't beat yourself up (I'm not necessarily saying you are), but you can't make somebody love you. And most likely, if they're the one who broke it off, it wasn't going well for them. It wasn't you.
If the situation was that it was a long relationship broken off, well, I'd need to know more to give you better and more specific advice.
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