Relationship advice!

Me and my boyfriend went on a break and I slept with another guy that was one of my exes. I kept denying it and he recently just found out a couple months later and he is really mad and hates me. I pleaded him to give me a week to prove myself because I love him endlessly and I know I messed up but he's the only guy I want. 
Does anyone have any ideas of how to prove myself in a week??? I have a car, and I have money but not a lot of it I only want to spend like 50 dollars cause that's all I really can