Danielle, you're letting him walk all over you. Sure he likes you, but not enough to have a relationship with you. He gets upset when you date someone else. You ask him about it and he gets angry because you're 'pressuring' him. Are you really so desperate as to accept this second rate treatment?
For what it's worth, you should never wait around for a guy who only likes you enough to give 'mixed signals'. Don't settle for anything less than a man who will move heaven and earth to be with you. If this gets upset that you move on, just tell him "Tough luck. You had a chance and told me that you weren't interested. So I moved on". Get some self respect happening here.
Go and listen to Beyoncé's "Put a ring on it" a few times.
I also want to add that you're NOT best friends. Best Friends are platonic with no agenda; what you've got is far more complicated.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.