Why can't I forget about them?

Because you're probably still grieving over the loss. Whatever that loss may be. Just let yourself go through the motions

Because Ive had too many good memories with them, its hard to forget 5 years of friendship because they messed up.

Because you don't want too, you are responsible for yourself and you have the power to do what you want to do with that brain, think about them or read a new book, its all upto you.

Out of sight, out of mind, right? It seems like without putting that into place (or even using that tactic!) it's easy for someone, without warning, to pop into your mind. One of the best ways I've found to get someone out of my head is eliminating anything that reminds me of them and when they come into my mind, I think about something else that'll occupy my thoughts for a while.

Most likely, because you are trying to forget about them. That isn't really how our brains work. In trying to forget about them, you are just thinking more thoughts about them. Your brain is very habit-based. If you think about a person a lot your brain interprets that as 'this is important. I need to think about this more'. If you get upset with yourself, your brain gets told that this thing you just thought is associated with intense emotions and therefore: "This is important. I need to think about this more". So, you need to break that cycle. Fill your time and mind with other things to think about. If you think about the person, don't beat yourself up about it. Just accept it, and then direct your mind to something else. Give it something else to make a habit with thinking about.