My old story of crush on somebody

I starting to like this boy in my last grade in high school, he is a nice guy, he also care about me. but since I know he already have a girl friend, I take a step back and never say it to him.
and I decide to be just his best friend, and it going well until now.
I've been around him for 4 years, and I know him pretty well, and the more I know about him, the more I fall in love with this guy.
but there's nothing I can do, he really loves his girl, and in his eyes, I just his best friend, not more.