Personally, on her bday I would dress up nice, maybe some new cologne, grab my coat, say "have a nice party", and just leave. If she asks where you're going just say "out, or I have other plans". If she asks when you will be back, say "I don't know". keep it short, say your late, walk out. Smile, be upbeat.
Go to dinner, a movie, a game, something else fun. Just get out and enjoy the night.
It seems rather crazy to attend a party celebrating someone who has tossed you to the curb.
Me:49 W:45 M:19 T:22 EA confirmed and ended 8/2014 S:19,17 D:9,5