I know what you mean. I have a desire to be a published book author (and I have published for pay), but I fear rejection and criticism. Your teacher is right that most all editors are ruthless and don't cut folks much slack. They can even reject something after only reading the first sentence, and indeed, they will not tolerate bad grammar.

Do you have a grammar book you can consult? Does the school offer such a course on-line?

I feel for you. I would be curled up, crying too. That said, though, I think she would be actually doing you a disservice if she weren't honest with you. Too many people have been told they write well and then keep at it when they don't stand a chance.

If your grammar/spelling is what is holding you back, then you can learn to overcome that problem.

Also, some of her ideas come from her own biases. As we know, fifty editors can reject a submission, and then one comes along and accepts it.

I suggest you try to hang in there. Is she the only professor who teaches the writing classes?

Meanwhile, I will stick my tongue out at her, for your sake: .