Feeling uncomfortable to even try to smile/laugh

I know i'm severley depressed when I literally feel an uncomfortable "burning" sensation when I attempt to smile. My laughter is very fake ans It's uncomfortable for me to even try and laugh. I keep thinking in my head "you can do this- just relax, let yourself go and enjoy yourself". I can pinpoint the last time i had a "belly laugh"-almost 3 months ago. (Oct 20, 2013). I don't think i've laughed at all in 3 months- maybe "sort of" laughed but i don't remember. I'm surprised more people with depression don't complain about the chronic lack of laughter in their lives. laughter is such a basic human function and when I can't even laugh, at all, it makes me feel like i'm insane. For a few months, I actually laughed a lot, everyday. I had spent about ten years before that rarely genuinely laughing. I maybe had a real laugh once every few months (from age 12-22). In ten years I probably genuinely laughed a dozen or so times. not much at all considering there were thousands of days in that time frame. Anyways, for some reason for the first time that i could remember, for most of the year that I was 22 years old, i laughed almost everyday. Sometimes i'd laugh hysterically and uncontrollably, it felt great, I felt alive for the first time that I could remember. I no longer considered myself depressed at all. perhaps it was a year long episode of mania? I don't know (never been diagnosed as bipolar). But who cares? I enjoyed my life during that year. And then, literally over a couple of weeks, the laughter dissapeared entirely. I reverted back to severe depression. (The depression came back in full force around September/October, and briefly dissapeared on Oct20, then came back and hasn't gone away, and feels like it's getting worse). I can't even fake it anymore. it's so exhausting and everyone knows that i'm pretending anyways.

How has a lack of laughter affected your life? was there ever a time in your life when you felt that you wern't depressed and you laughed a lot?