Yes, yes you are. And even you think you are!
Look at your excuses for not going to your husband's family's event:
Of COURSE it's important to your husband - it's his parents, it's something they've never asked for before, they're hosting.... of course it is important to them, and by extension to your husband.
If it helps, I too am introverted, and my husband comes from a large family, and they did a big anniversary party, out of town, with EVERYONE (all siblings, all their children- 25 people). You know what I did? I sucked it up. When my husband could let me bow out for a meal or a tour of a local attraction, he did. And I had major issues with my MIL AND had just had a miscarriage (with D&C) 2 days before departing, which no one knew about. So as bad as you feel - you can rise above it.
Saying your "plans to get your passport" didn't allow this to fit in - you're grabbing at straws.
Do this for your husband and your marriage. It is worth it.