Why am I so stupid? WHY! HELP ME PLEASE (Dating issue)?

Ok. There is a girl (let's call her A) that I am in love with, but I am and always too afraid to ask her out. I really love her. I know the likes me back but I'm afraid she'll reject me. Well tonight I was hanging out with my friends at a concert.. Then these two SMOKIN' HOT blondes comes up to me. One of them says "Will you go out with my sister (Let's call her b)?". I paused for a second. I knew the James I've known for 15 years would say "HELL YES" but I started to think about A. So I said no. She said why not. I said I already had a girlfriend. I really don't. So they walked away. B was sad and her sister was angry at me. I know now that I still don't have A by my side, and I gave up a hot blonde. A's not ugly, she's actually pretty, but not model pretty. Don't mistake me I'm not a pervert or a guy who only looks for looks. I like girls with good personalities and book smarts. I'll never see B again and I still love A and afraid to tell her. What would you do in that situation?