why can't i seem to forget about him?

It takes time to go through the healing process after a breakup or anything related to that really. It will take time. Focus on other things.

You probably feel that you can't forget about him because he was a big part of your life and that is completely normal. I remember struggling with past relationships thinking I would never get through it, but some how I found a way to move on in a healthy way. It's not a bad thing to remember the good things about him. Those are fun and healthy memories right? He was a part of your life and that is something that you should think about. Don't dwell so much on the negative. Think about healthy ways to move on. Maybe you have some hobbies that you forgot you liked? Maybe you have some projects that you might have put on the back burner. Think of things that will keep you busy but also happy.