I need help. How do I handle loving my straight best friend?

I'm 18 years old and have known him for 6 years. He has been there for me through everything and I've had these feeling for him for a year. He doesn't know I'm gay, but I know he is fully straight. He has a girlfriend of 2 years, and the more and more I see him the more I want to come out, and tell him. My main problem is his rejection, making things awkward between each other. What do I do if I don't want him to know, do I try to slowly distance myself? How do I handle this situation?? PLEASE HELP!!!!!