Dunno how to cheer you up but I think you might be over-reacting just a smidge. I'm sure he could have arranged to meet this girl some other time and it sucks that he's choosing her over your 21st but it could be worse!
Last year I planned to go on one of those team building adventure day thingummy's with a mate. Archery, abseiling, raft building and the like. The company were offering a fantastic one off deal. It was tremendously discounted and completely free if you were unemployed or in receipt of certain benefits.
His girlfriend didn't want to go, so my mate only booked two places. At the last minute she decided she did want to go after all and my mate gave her my place. I made a couple calls but by that time they were completely booked up. I can understand why he chose to keep her sweet. He knew I'd forgive him eventually but she'd nag him every day for 6 months straight and still not forget it. It pissed me off but we stayed mates. Good friends can be tough to find.