I go through that all the time. Some days I wonder if I have a personalitiy disorder or somthing because it happens so fast. I will be so happy and cheerful and then bam! next thing I know I am totally down in the dumps. It sucks and ussually not much will get me out of it fast. I have been tyring to avoid giving into these feelings by not binge eating and not giving up on things. Some times i use it as an out not to finish something or start something.
You arn't the ony one, and I'm sure grad school isn't a picinic in the park everyday either. I went to grad school right after undergrad and I was so miserable, I was completly burnt out, and should have waited. I moved out of my house with my friends and got an appartment, piggedout on food, gainded weight, got pregnant, and stoped going to class ( I think some of it was the hormones from being prego). I'm suprised I even graduated. So remeber, you arn't as bad as you feel right now, you are as good as you were feeling earlier