I'm going to forget my girlfriends birthday due to sleeping in all day to catch up on sleep
Why do you want to sleep in?
I am so tired, I cannot miss anymore sleep this month.
You should make time around your partners event to get some sleep. People expect you to be supportive of your partner.
I guess that is where I differ. I am selfish at times. I shouldn't be but it is what it is until she says something about it.
You can't wait until she says something about it, then you will think she's nagging. Just be a decent person and try your best to make her happy on her birthday, otherwise you will experience unnecessary conflict with her and jeopardize the relationship.