How do you kick yourself into gear after (while you're in?) a bout of depression?

Hi all. This is a throwaway account, a little bit like some parts of my life seem to be right now. I've kind of drifted from a lull into a bad spell into a lost 2 months into a lost 4 months, and now it's really starting to get bad. Any advice for how to reverse this? How do you snap yourself into gear?

I'm in grad school, I defended my thesis but have to make some corrections and finish some classes before I can graduate in at the end of this semester. Instead of making these changes to my thesis, I traveled to a conference and took basically a mental vacation- came to work every day but just read the internet instead of accomplishing anything. Told my boyfriend when I would come home that I had done things when I hadn't. Failed to make follow-up contact with colleagues I met at my conference. Failed to apply for a PhD like my advisor suggested. Broke up with my boyfriend because I felt like shit. Continued coming to work doing nothing. Heard about some family difficulties. Freaked out. Have not applied for jobs. Have not updated my thesis.

There are islands of my life where I still do things right- my friends from old towns don't know anything's wrong, I am in a (non-scholastic) club where I show up and am enthusiastic and everyone thinks well of me, I have one consulting gig on the side that has continued to go well. I started seeing a new guy and he didn't know anything was wrong (but I called that off on Monday).

But lately I stopped even going to work pretending to work. I am now 3 weeks behind in homework in one class, and 2 weeks behind in another. Haven't gone to a lecture since last Wednesday. My coworker sent me an email last night asking if I was all right, told me "[I] look lost, somehow." I need to snap out of this, and NOW, if I am going to graduate.

How do you guys do it, when you get in downward spirals like this? What brings you to the surface?

I'm ready and willing to answer any questions, but I AM going to go to lecture at 3 and be in a lab til 7 (EST). Please, any advice you can give will be taken to heart. Even if it's cruel.

Update 1: Thanks so far, I'm going to class! Please keep up the advice, and thank you very much. Update 2: Fifteen minute break between class and lab, I'll respond to what I can, and I'm really proud that I made it to class. Hopefully gym right after I answer your questions after lab.