In need of advice I don't know what to do

I dated this guy for a year and a half, there were some parts of our relationship that I couldn't understand and he recently told me why it happened that way, I'm really upset because I feel like I could have changed somthing, it's been 3 months since we broke up. I saw him recently and it completely fucked up any progress I made, he went on a date this week and it looks like they are going to get together, I can't make it stop hurting and I want him back and I don't know how to make it stop or what to do. Any advice is welcome, ik I can't stop them and he has the right to get another girlfriend but he just means a lot to me.