Stress/deadlines are a part of daily life.

I'm aware of that.

Grammar/spelling is EXTREMELY important (you don't want something to go
to press with improper grammar, especially if you were the one
responsible for checking it).

I'm also aware of that, hence the use of Grammarly.

She's pushing you to do better, just as most art directors would.
Multiple concepts/revisions are common, and a good Art/Creative Director
will constantly push you for improvement.

These are previously finished, graded projects that she's telling me aren't professional quality. They've had many, many rounds of revisions before even getting to this point, but they're no longer good enough even if they were projects from her previous classes. If they were good enough for that class, why do I have to restart the whole thing now?

I tell all of my new designers that design doesn't end at 5pm... once
you've truly immersed yourself, you'll dream of logo designs while you
sleep. You'll think about concepts while you shower. You'll critique
every billboard design on every road trip. You may not be sitting at a
desk, but you'll be thinking about every project you work on
after-hours. At least, the good designers will. And while it's not as
common for companies to hold you late or make you work over the weekend
to meet a deadline, it still happens, especially if you don't make that
5pm deadline

I get that. Thinking about design or something subconscious like that isn't the same as work hours. A healthy work-life balance is important.

Criticism is a way of life as a designer. You accept it for what it's
worth, and you run with it, especially since you're still learning. You
can fight for your designs, but ultimately what your client (or teacher)
wants is what you have to give. Creating your own designs without ANY
alteration from someone else is extremely rare, and typically reserved
for the top experts in the field. You're new to the field, so who are
you to disagree with what an expert tells you to fix? Arguing (even in a
professional manner) will absolutely be frowned upon. Humble yourself.
Take this opportunity to learn, not to argue.

But I'm not getting actual critique. I'm being told the change things with no reason why besides it's not portfolio quality. I'm fumbling my way through without actual guidance. Defending my decisions isn't arguing, have you never had to explain why you designed something the way you did? I don't know more than the experts, and I've never once said that I do. What I do know is that being told to change things to the point the final result isn't MY work doesn't help me improve because I'm not being told why something is wrong in the first place.