Trust takes time, usually a lot of time. But I think a more important question to work on while you're earning his trust back is why you didn't just block this guy when he stepped over the line to begin with. Why didn't you just end the relationship then and there? What made you think you had to participate in an elaborate farce to keep this other man happy? How far would this have gone if it had not come to a head in this manner? What if this other guy would have shown up on your door-step when he knew you were alone? This is dangerous behavior to your own safety.

If you don't figure out why you did something you were uncomfortable and didn't want to do to begin with, it'll happen again. There's a much bigger problem than losing your boyfriend's trust.

Couples counseling&why was this guy who supposedly was your bf's friend hitting on you or communicating w/you in the first place? Like the last responder-trust is hard to get back-I've lost trust when I didn't even do what I was accused of&no matter how much I tried to defend myself my bf wouldn't take my word for it. Find out what exactly your bf has in mind as far as how you can earn his trust back. Don't do anything that will degrade you-he'll just push you around if you do-but I see no harm in giving trying to re-unite, if he really feels the way he says he does. Ask him to question whether he'd be more likely to trust something you've said or his supposed friend said-who is he closer to?

Give him your phone and tell him to break it, symbolically. You can get a new one. Or just throw away your SIM card from your phone. Do it in front of him as a way to prove how much he really means to you and that you would make any sacrifice.
I don't know if trust can be fixed. Sometimes it can and other times it cannot. Give it a shot by talking to him and get a feeling on how things are going.
Good luck.