Well I think at this point YWBTA if you backed out. Your girlfriend brought this trip up to you months ago, and instead of communicating your concerns in the moment, you festered resentments about it. Also, if your GF is very close to her mom, I don’t think it’s particularly weird she would want you two to get to know one another sooner rather than later. Everyone is different, and there is no one correct timeline in which to meet relationship milestones. Ask yourself this: if it was her sister or friend, would you feel as awkward? Perhaps her mom is super fun. Why do you feel like you’d be interrogated the whole time? Is that a reasonable assumption or is that perhaps a personal fear or bias?

I’m super introverted, dread forced social interaction, and get stressed out about high stakes social situations. But I would never back out of a months in advance planned trip with my SO and his/her parent. Nor would I resent my SO for very kindly including me (and paying for!) a vacation that is clearly important to them. I either would decline upon invitation or shortly thereafter, or I would suck up my fears and insecurities and just go and try to have a good time, which is usually what happens when I force myself out of my comfort zone.