Chances are if you can't make the other work, you can always go back to this job. My partner doesn't like this sort of thing. Although I welcome her presence. I feel I should be able to go alone, to be with friends and have the opportunity to meet new ones. Voting for Dinah A. Dear Voting, I would not want to be in your shoes for anything.
Ultimatums have no place in an intimate relationship.
Your girlfriend seems very controlling and she uses that weapon as manipulation. If the two of you have been in a serious relationship for over a year, and there have been no major issues, then, you each should have enough trust in one another, that a weekend apart should not create such turmoil. You offered her the opportunity to go with you, what more can she ask for. I say pack those bags and have a blast in Palm Springs. Sounds like your in a predicament.