I'm thinking of uninviting my girlfriend from a family vacation
Why do you want to exclude her from the family vacation?
She took the last pop-tart and ate it for breakfast.
If your girlfriend is being rude, it is fine not to invite her on the family trip. That having been said, you should not leave her out just because of some minor issues. Your gf needs to be included in family gatherings.
I'm not sure if I should forgive her or not. I paid for that pop-tart.
Your gf eating the last pop-tart is so trivial as to not even be a real reason to uninvite her from the family vacation. She deserves a place on the trip, so let go of this unimportant thing and bring her along.