Document, as already stated. But as an aide, do you chart notes?
Do keep your own record and do an incident report and keep a copy. Even if they tell you you aren't allowed to keep a copy, you can still write down the pt's name, the date, her statements, etc., who told you to do what and then deserted you, it sounds like, didn't help you with this extremely large patient, did PT put her into a chair and leave her there in a safe way, was her call bell handy, were wheels locked, was the chair at the right height, and all details, lighting levels, any obstacles on the floor, etc. Be thorough.
And no, you can't avoid all mentally ill persons. This patient needs therapy, maybe medication, for depression and anger.
Be strong. Go back to clinicals. God bless you.