It kinda sounds like you never agreed to make it a committed, exclusive relationship. It also kinda sounds like he waited around for you for 5 years to come around and realize that you wanted him as a boyfriend. Did you ever have a solid conversation about you and him being boyfriend and girlfriend/partners?

You keep talking about how he had feelings for you and how much he was into you, but you don't really talk about how into him you were or how willing to commit to a relationship you were.

These details are important, because if you denied him when he asked you to be his girlfriend, then he absolutely did not cheat on you, and it's good time that he moved on. It doesn't matter that this other girl knew that you and him had a thing. If my assumption is true, then it doesn't matter how long your history. He was a single man.

From what it sounds like, you've been leading him on for 5 years and he's finally gotten some respect for himself and found someone that is willing to commit to him. You never call him your boyfriend or your partner, and nowhere in your post do you say you're anything more than friends that care about each other, and you never say that you're in love with him.

It seems like you didn't really want to date him, and he's been wanting that for a long time.

Even if my assumptions are wrong, take this as a life lesson. If you have romantic feelings for someone, confess them and ask for a committed relationship. Tell people how you feel, and if they don't return your feelings move on. If someone tells you they want to date you and you're not sure, just say no and let them move on. It sucks that you lost your best friend, but you handled this entire situation poorly, and all you can do is pick yourself up and realize that if after 5 years you didn't want to commit to him, then he wasn't the guy for you. If you can't handle seeing him, then don't see him and find new friends. Take this time to work on yourself and find happiness by yourself.