I'm sorry you're having problems with this particular teacher. I do echo what everyone else here has said about grammar and the ruthlessness of editors in the writing world. Is there a grammar clinic you can go to? Or have someone proof-read for grammar before you submit your work? If not, there are a couple of good and easy-to-use grammar handbooks and guides out there that may help.

But I am sorry this is so hard for you right now. In her eyes, she may only be trying to help. I don't know her or the specifics of course, so I may be wrong. Is there the option of another instructor?