Thanks for the update. I'm sorry you got a saddening letter and that your school work has been so effected by your depression.
Know that it doesn't have to last forever, though it may feel that way right now.
It may be a long, tough road, but you seem to be really smart and know some things that may help get you thru this.
I think a diary is a great start. It might help you to see patterns in your thinking or even bring out things you aren't aware are on your mind (if that makes sense).
Good luck and here are some big for you.

BJ