I'd say you're having a massive panic because you've been conditioned to put everyone else's wants first before your own needs. Standing up for yourself when you've had that conditioning causes you to panic at the consequences of putting yourself first.
Logically you know you've done the right thing, the only thing that's reasonable to do. But because you've been trained that it's "wrong" to ever put your own wellbeing first, and there's always been consequences (punishment) if you've stood up for yourself before, your brain (which hasn't adjusted to your new totally normal and reasonable life rules yet and is still in victim mode) is expecting punishment and that's scaring you.
The way to avoid consequences in the past would be to backtrack, back down, "put things right" by apologising and return to how things were ie you at the bottom of the pile. Your brain has incorrectly labelled your current actions of being assertive as "wrong" because it's still in victim mode, so now you're panicking in the way you would if you'd just done something actually wrong eg a hit-and-run in the car or mugged someone.
Basically OP, it's a trauma response and these reactions will subside over time to become less frequent and less severe.
Your parents are arseholes. You've escaped an abusive relationship. They should be
A) believing you
B) giving you a big hug and comforting you
C) pleased you've escaped
D) supporting you emotionally and practically in any way they can
They should not be
A) dumping their emotions on you for you to "fix" as if that's your responsibility
B) unhappy that you've left a toxic abusive situation
C) siding with your ex in any way
D) disbelieving you at all
That they are doing any of these things means they're toxic themselves. You might want some therapy to deal with that and recover from it so you can move in with your life properly, free from the kind of emotional baggage that could see you landing back in another abusive relationship if you start dating again.
Your DC only saw what you two showed them. Obviously you'd have tried to protect them from the worst of the DA and they're also children, so they haven't known all that's happened to you and they don't really understand the nuances of adult relationships at this stage in their development anyway.
There's no pleasing DC a lot of the time. Even in cases where horrific child abuse or neglect has occurred in a family and the DC are removed into care, most of them aren't happy about it. Because they didn't ever really want to be removed from their parents, they wanted their parents to be people capable of loving them and caring appropriately for them.
Your situation isn't one of CA but it also isn't so different from a child's perspective. They want to be a family, they want their parents together, they want the marriage to be a happy one. And if wishes were horses, I'd have a whole herd. But children have yet to learn this. All they know is their family has fallen apart and they aren't happy.
They're entitled to their feelings about the situation. It doesn't mean you should have done anything differently OP or that you've done anything wrong. It also doesn't mean you should backtrack to appease DC or try to "fix" their emotions for them.
It's ok for people to experience negative emotions, it's not ok to martyr yourself to prevent others from having to experience and deal with their emotions. As an abused parent you couldn't have been operating in full capacity as a good parent. Now you've fixed that issue and can be a parent to your full potential. You can give DC at least one home where there's no undercurrent of abusive behaviour and the toxic atmosphere that causes. That's a good thing.
Don't forget also there's possibly some parental alienation going on by your ex. And all your DC, including the oldest, have been harmed to some extent by living in a DA household where such behaviour becomes normalised. Your oldest kicking back against the marriage breaking up is a sign of that. As he matures he will hopefully be able to see that remaining in an abusive situation wasn't a reasonable thing to want his mum to do.