I cheated on my boyfriend, now he wants to dump me

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My boyfriend and I have been having emotional problems for a few months.  One day my best male friend was comforting me, as I was upset and scared that I was losing my boyfriend.  One thing led to another and I ended up sleeping with him.  I’ve told my boyfriend what happened and he’s seriously mad at me and is thinking about ending our relationship.  I really love him and I want him to be able to trust me. What can I do to prove to him I won’t do it again and that he can trust me no matter what?