You are not alone.
This time of year is tough for me even when I'm in good spirits. Wrought makes a very important point about it being temporary.
Volunteering can be one of the quickest way to add some contact with people thankful for your help. As a recluse with family not living nearby, volunteering's additional human contact is a big portion for me personally. Thebrick gives some great examples, as long as it is something that you can identify with.
Not sure if this is appropriate to your specific situation, but I thought I'd add it. Some years ago, I had a bit of a breakdown at the end of the year. I think the end of the year is also a natural mental divider of the year when you tend to reflect back on the past year and look forward to the next year. At that time, I felt that I was in a dead end job which barely paid the bills and felt I had nothing to look forward to. I was also spending about 4 to 5 hours after work studying world politics. *sigh* That really isn't a great topic to make you feel better about the world. Near the end of the year, I decided that I needed to address this issue. I tried to face it all at once and cracked. I checked out of life emotionally and mentally. To be frank, it was really bad for me; however, it was not that bad compared to some people experiences - I never tried to hurt myself and I didn't abuse substances too much. Eventually I came around and took some baby steps towards addressing those issues. My point is that sometimes the end of year depression has some roots in personal reflections of what we perceive, at the time, of where we have been and where we are going. Large paragraph before getting to the point, eh?
Definitely continue with therapy. I've gone before and it has always helped me. In fact, I'm looking for one now. And if you think you need more help, please don't hesitate to visit and MD. Much (all?) of the emotionals inside our head is chemical interaction. Sometimes those chemical ratio are off and need correcting.