I absolutely love NVC and found this to be a great primer:
https://medium.com/s/please-advise/the-essential-guide-to-di...However, NVC mainly talks about _giving_ constructive criticism, not about _receiving unconstructive_ one and I still missed that piece for my own assertiveness until I recently found something that clicked with me on that part:
I got some great insights from the book "When I say No, I feel guilty" that were especially easy to put in practice and helped me to massively boost my own assertiveness and to me is an extremely valuable extension to NVC on dealing with attacks on the own assertiveness.
The book goes into much more detail, but I can sum up the essence in just three paragraph here:
You have the fundamental right to be your own judge on everything. That includes being wrong, illogical or changing your opinion. Now how do you put that in practice if someone wants to impress their opinion on you?
First, stay calm, friendly and agree with something that they said — and if that something is just their own feeling! (They call it "fogging" as if trying to hit a fog bank)
Second, calmly stay with your opinion. ("broken record"). Don't stop until the other side has given up. Never explode, never yell. And that's already it.
I found that combo to be extremely effective in practice, because you don't actually give any attack surface. Here's a sample dialogue:
A: I think you should go to bed earlier, otherwise you get wrinkles.
B: I agree it's good to go to bed early, but I don't want to.
A: Come on, wrinkles would be ugly on you!
B: I see how you might feel that way, but I don't want to.
A: It would make me very sad to see you with wrinkles.
B: I appreciate you caring for my appearance, but I don't want to.
[...]
Obviously hair-pulled example, but you get the picture.
Works A-OK for me, it quickly entered my daily conversation.