OP, I'm sorry you're going through this. You're very articulate and perceptive about the nature of your response, and the thoughts and feelings that come up for you, and that's massively in your favour as it will help you process new ways to handle this.
There's a life coaching model which I think is based on CBT, which might be useful. When I've used it, it's helped - i need to use it more!
It goes: circumstances trigger your thoughts. Thoughts drive your feelings. Your feelings drive your actions. Your actions shape your results.
So your model might go:
Circumstances: brother says something critical
Thoughts: I'm letting everyone down/this will haunt me forever
Feelings: fear, anxiety, doubt
Action: appease brother/change plans (what is the action?)
Result: you don't enjoy the event/you change what you had intended to do
The trick, which has to be learned, is to ignore the feeling but change the thought.
The feeling is what's causing you pain - that shock fear punch to the stomach- so it's natural to focus on that. But it won't change without the thought changing, as the feeling is really in response to your thoughts, not the circumstances themselves.
And the trick to changing the thought is to come up with a believable alternative. You won't be able to get the results without believing your thoughts, so replacing "I'm letting everyone down" with "everyone will love this" when your brother criticises isn't believable. But, thinking "he's being grumpy" might be easier to believe. Then your model becomes more like:
Circumstances: brother says something critical
Thoughts: he's being grumpy
Feelings: mild annoyance/compassion
Action: shrug, invite constructive input, move ahead with plan
Result: event goes ahead as planned
Sorry for length - this helps me when I remember to do it! Best if you actually write it down in a way that's true for you