my bf cheated on me

My boyfriend cheated on me back in October with a girl that I knew and was friends with in high school  and I took him back after a month of not being together. Things have been up and down between us we go periods where we are really good and periods where we fight about what he did. I try so hard to get over it but it's hard, I have noticed a change in him for the better but I'm always constantly worried it's going to happen again. I can't move past it but I really see he's trying I just always question can a person change? Or should I just leave?