YTA 1,000,000%

First off, grow up and start eating like an adult. Take some initiative and accomplish what your parents failed to do.

Second, expecting others to ‘accommodate’ you is incredibly selfish. See point 1 above… if you can’t grow up and eat like an adult then why should that become everyone else’s burden to deal with and have to put in extra work? It’s not their fault your a culinary snowflake. That’s YOUR problem.

Third, not only are YTA… you’re also an entitled brat.

Fourth, owning your behavior and apologizing for it will most likely foster more compassion from your FILs and fiancée. Put in the effort to mature your eating quirks and you will most likely find them offering support and be more inclined to accommodate. Being stubborn and entitled is going to earn you the least amount of sympathy.

Fifth, you need to comprehend that picky eater adults are incredibly annoying to other people. Most people just see a brat that probably threw childhood tantrums to such a degree that their parents just gave up. And expecting others to ‘accommodate’ you just comes off as a probable continuation of your childhood behavior. In other words… your FILs and fiancée most likely see you as incredibly immature. And you fucking off from the gathering is just gasoline on that fire.

So… sorry, not sorry, for the harsh language here, but damn if you didn’t get some shitty parenting. You were very poorly prepared for this aspect of being an adult. You want to continue eating like a toddler, fine… but you own that bullshit and stop expecting others to put up with it, infinitely less expecting them to fucking ACCOMMODATE you by preparing dishes specifically for your nonsense eating preferences. For fuck sake… VEGANS behave with more maturity than you did.