Lostboy saidYou know, the problem is not her. It´s your boyfriend.
Exactly. Not that her behavior is a joy, but it's how he
feels that will or won't be a problem.
No point in being jealous though. As has also been said, she's known him longer so their relationship will have aspects yours doesn't yet. That's not your fault or her credit, just happenstance. Letting it bother you will just bring you down to her level, and if he TRULY is tired of her immaturity, he won't like it in you either, so stick to the high road.
But your question is how. Only thing I can offer so far is "fake it till you feel it". If you pretend it doesn't bother you long enough hopefully it won't. And if you relationship continues to grow, you'll realize she's not a problem after all. Or she'll finally go too far and he'll dump her.
Or, sorry to say, it will actually be a problem and you'll get out of the situation.
About New Year's Eve. I think you have a right to know if you get to spend it with your bf. Not a right to do so, but a right to know if you will, so you can make other plans if not. Choosing between a longtime friend and new sex partner** is not easy and you shouldn't make it a bigger issue than it is, for both your sake and his. So go ahead and ask. It really shouldn't shatter you if he chooses her this time, it's just a general populace party night, it's not your anniversary! Look at the big picture of how much time you each get, not at the specifics of NYE.
Also remember it's not his fault you and she don't get along.
** you haven't said how long you've been in this relationship, but obviously it's new compared to them.