It sounds like you're speaking out of frustration more than laying out the order of things.

It seems to me that they've hired you as a writer - paying you what you call "ridiculous sums of money" - not to mess with you but because those who hired you aren't writers. They don't know better; this is where you come in. And you did come in: if your words are anything to go by, you did write some "AMAZING SHIT".

From then on, there are two things that you might do.

One is to consult the people who's changed the things you wrote on why you wrote what you did and why it's better than what it ended up as. This is a path of frustration, because now you have to explain 20+ years of experience to someone who's by far less skilled and expereinced in about five minutes or so, and it will not be a pleasant endeavour. If you care so much about your work, you might want to take it and, whatever the result, leave with the idea that at least you've tried.

Or, you might come to realize that you've done your job - and beyond that, nothing is your responsibility. They can make it shitty; they can leave it as it is; hell, maybe it will just happen that they improve upon it. If there is something to learn from, you might want to do so, but after that, there's not much you can do about it - nor should you. You've been payed not to babysit someone but to write stuff; you did. That's it. You're done, and perhaps you've even done well with that, but that's as much as you have to do.

Doing what one loves will probably entail a lot of stress - but not because other people are assholes or some other such shit. It will because those who do follow their dreams care about what they do and believe - with a degree of righteousness - that what they do is much better than what others might do. Just because someone pours Coca Cola over the amazing whisky that I've made doesn't make my work any less or the drink any worse - it just makes for a person who can't or doesn't want to have a refined taste for alcoholic beverages. And fuck tearing myself a new one over that poor schmuck: that isn't worth it. I'd rather enjoy the fact that I did good, no matter the result, because it's not the result that you're going to live with - it's your part in it.