I feel kinda slighted that my gf wants to spend her bday alone instead of doing something special with me.
How did you show her that you're slighted? Or did you even bother?
She did it before for her 28th birthday.
it is not normal for people to spend their birthday alone. You should ask why she wants to be alone and also respect her wishes.
I'll just leave her alone. She promised that we will spend our 1st year anniversary together.
That's probably a good idea. I'm sure she looks forward to being alone and relaxing for her special day.