To add to this, something that helped me accept this change: your acquisition marked the achievement of your startup goal. You're done now, you can now stop living for your work, and start working to live.
I think a lot the advice in this thread about navigating large company politics is missing the point a bit. I've heard entrepreneurs and people who like to work at start ups describe they don't do so because they necessarily like to, but because they have to. They are either bad at, or can't/refuse to deal with large company politics so head in a different direction.
Unless you're employee ~3, you're probably not going to make a huge financial windfall. The only counterexample I've seen was from an employee who invested in startups themselves. (Apparently he even "investment sniped" one that later became a YC co, which I found pretty impressive.) Suffice to say, he knew how to play the game.
I think lots of people work at startups because they like to work. There's nothing wrong with that. But don't be too surprised at the end of it when you walk with a total comp of less than you'd have made at $bigco. People work at $bigco for the money because it sucks -- that' why it pays $bigcash -- and OP's story shows the kind of pain you're avoiding by taking a pay cut.
OP, I wish you all the best. If this is your first experience with a big company, well... Just know that it's far worse at big banks. :) You're lucky in that sense.
When you win the lotto you don't toss away the ticket so you can play again. Even if OP didn't cash out founder style - they still get essentially a multi-year paid vacation.