how to get over the "ex found someone else" thing

20M here, to make a long story short, I have recently been trying to reconnect with my ex. i broke up w her b/c i was going back to school in another state and i feel like i made a mistake. Unfortunately I have found out that shes been seeing someone whom I have had a bad falling out with now. Thinking about them together is terrible. All i keep thinking is how better she is than him. Anyone have experience with this or went through this recently? I don't usually post in these but its been a rough patch for me recently in my life.