I know you don't want to tell your bf what happened but you should really do so, so you can both work out what needs to be done to fix this problem. If you keep this a secret not only are you lying to the poor fucker and not allowing him to decide if you're the right partner for him, but you're also going to be stuck in this loop with Mr XYZ which could eventually lead to disaster.

Also the whole “I'll never cheat again.“ thing is bullshit as I'm sure you thought that you'd never cheat right up until the point that you did...