It can be overcome but the fact that you shitted on your agreement to be life partners I.e. Engaged sorry but he should avoid u. 

But if your lucky and your stars are aligned he might be able to move on with you but that is a huge ask

It will get better eventully. If you want a fast and easy answer though, end the relationship, there is no fast and easy way to regain trust after you break it. It might take months, maybe even years, it wont be easy, you will be reminded of it countless times, and feel guilty about it often. If you want to earn back his trust then you have to think from his perspective, his trust has just been betrayed, it will take a while for him to build tht trust back up. It is good that you were honest with him, just continue being honest and try to show him everyday that you regret it and wont do it again. If you go out drinking, keep him posted on what you are doing. Some of the other people are being rude, but dont worry too much about them, yes you did a bad thing, but it can be fixed with enough work. It will be a long hard road to fully regain his trust, but I believe you can do it if you try. I hope you made sure to emphasize that it was a mistake and that you would never do it again, because that is important. One thing you can do is offer to let him sleep with another woman, but only once, hopefully he would say no, but if he agrees to do it, then you have to endure the pain you put him through, and he has to endure the shame and guilt you went through. As I said before however, there is no quick fix, it will take a lot of time to make these scars fade enough for you both to forget about them entirely.

:-P

You hold no accountability, and until you learn to have some you will make more choices like this. I choose the word choice because that is exactly what it is, it was not a mistake. A mistake is when you didn't mean to do something and had real positive intentions to do something else, however, lack of knowledge or otherwise something out of your control changed that choice.

You drank, went to a rave, engaged in behavior with another man that was going to what? Do you believe the other man was there to discuss the latest season of Game of Thrones and who will get killed on the show next? No, he wanted what you gave him. You, on the other had was seeking what? That enviroment is for exactly what you did, get drunk and have sex with someone.

You drank, nobody poured it down your throat. You are to blame and you made a choice to do what you did.

You are shelfish and are looking for every excuse you can to not blame yourself. Your friends, drinking, the other guy, raves....the point is this. You do not respect other people, you only care about yourself.....or did you ask the guy at the rave if he had a girlfriend before you kissed him?

Ask him what will make things better again