I have the same issue, but I work with the public (work at a bank) and sometimes it's so hard to make clients feel welcome when all I want to do is cry.
Sometimes I'm able to get with one of my employees and laugh about something. I have one that knows my moods, and she knows when to do something funny, or start talking about something that makes me happy. I like the other poster's suggestion of a happy picture.
If all else fails.... try chocolate