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.