My boyfriend's best friend is so amazing that I have a crush on him, and I have no idea how to deal with it.
Not your boyfriend? Why your boyfriend's best friend?
Probably because he is so handsome
You need to break up with your boyfriend so that you are not cheating on him. If you have feelings for someone else then you should be honest with your boyfriend so that you are not lying to him. If you are not honest, your boyfriend will break up with you.
I can't tell my boyfriend, though. I want to get married to him when we are older. It's just a crush.
It's not just a crush; it's being emotionally unfaithful. It's one thing to find someone attractive, but it's another to have a real attraction to someone so close to you and your boyfriend. Lying to your boyfriend about it on top of having that crush is dishonest and unfair to him.