I might be mellowing in my old age but I just don't think this is worth getting into with someone who won't be your roommate in a month.

Absolutely it's inconsiderate and rude, but it's also not your problem in about 28 days.

You've told her it's not cool, not much more that can be done. She's not going to suddenly realise she's being a total pain in the ass and apologize. All she could realistically do at this point is move out, and she is.

Stay with your gf as often as you can, or have yours around so you're still getting 50/50 on expenses.

Go away for one of the weekends. Easter if you can.

I don't think this is going to be an issue going forward because you aren't going to be friends after this.