Something I'd like to nip in the bud, if I may. Whenever these threads come up, I always cringe when people start in with the, "Toughen up, or get lost," mentality, or, they assume that all writers are super fragile and must be handled with kid-gloves. Some? Sure! But, for the most part, no.
Sometimes, there are assholes who act like assholes when they critique. Let's not forget that.
Teaching writing is advocacy in a sense and I find that I have to help a student repair damage to the inner writer in 8 out of 10 cases. Not because they are quivering masses of emotion, ready to jump off a cliff, but because they've had people with no tact tell them to give up along the way. Glad that many of them did not because they ended up being pretty dang amazing writers.
Whenever I hear generalizations that because someone is asking how to thicken their skin, they must be mentally ill and unprepared to face the world outside, I think of people who either have no tact (and need to blame someone else for it), or, I think if someone who just doesn't understand that there's a lot more to it than that.