How do I gain back my reputation and my boyfriends trust?

Hi there! This is a personal question. In order for you to understand, Im going to provide as much personal details as I can. Please, feel free to discard this question if its not of your interest.
Well, last summer I made a huge mistake. It involved alcohol( lots and lots) and well.. another guy. We just kissed… but because we were both in Europe for the summer, we remained good friends, though I was certain he wanted to be more than friends.
This is the hard part: I have a boyfriend. I love my boyfriend, I really do. I love him with all my heart. It was just a stupid mistake and I swore it would never ever happen again, and I didnt tell him. I thought it would just go away.
How stupid of me to think that, because this guy I made out with told EVERYONE. All our friends and acquaintances. I just wanted to ignore it and I prayed my boyfriend never found out. That didnt happen. I stopped having any kind of contact with this guy or his friends.( The guy does not live in the same city I do, but we still have lots of friends in common because we went to the same highschool, as well as my boyfriend as his friends)
My boyfriend found out eight months later. It was absolutely devastating for him and I felt terrible, how I could ever do that to him? After lots of hardwork, he forgave me and gave me another chance.
But now, all his friends think im a mayor sl**t, wh**e, etc. One of his friends actually told him how I made out with other boys and kept cheating on him( which is absolutely not true). And thats all I know about… there could be millions of other rumors about me.
And worst of all, I lost my boyfriend’s trust. Though Im starting to slowly gain it back… it looks like a long way to go.
Do you have any recommendations or ideas on what should I do with this whole situation? My friends tell me not to worry, that it will pass, but I need a second opinion…
How do it gain back my reputation, and how do I gain back my boyfriend’s trust?
If you read all the way through here, I really appreciate it. Thank you.