My friend is dating my ex which i want back, help?

So i dated this guy and we went out for like two weeks then we went to be freinds with benfits for months. and so i move on and so does he but he moves to this friend of mine, we aren’t besties but were aren’t enemies. and i want him back like crazy and she is mad about it with me but i want him back like no tommrow. and i need to know how to get him back? HELP.