Identify the Issue(s)
If you can't change the situation, the only option is to change yourself. Staying positive at work is not about ignoring the problems you see; it's about changing the way you see them.
For born optimists, approaching issues at work from a positive, constructive standpoint can be automatic. They likely approach everything that way. For those new to optimism, though, a shift into positive thinking starts with some analysis: What am I feeling negative about, why do I feel that way about it, and how else could I be looking at it?
Unfortunately, if negativity regularly defines your experience at work, individual issues may have morphed into a giant, nebulous ball of wanting to call in sick. This is difficult to work with. If you take a step back, though -- and journaling can help with this -- you may begin to see some concrete triggers. For instance, is the company financially unstable, and you're concerned about lay-offs? Has a "failure" in your work led to insecurity about your own competence and/or job security? Are you troubled by your interactions with the boss, employees and/or peers? Has your role in the company changed, leaving you feeling uncertain or ill-prepared?
The next time you find yourself sinking (deeper) into the negativity hole, take a moment, remove yourself from the situation as much as possible, and take some notes. It will help you sort through the noise and figure out what's actually going on in your head.
Job insecurity, dysfunctional workplace dynamics, perceived failures, uncomfortable changes in the status quo -- these are all excellent at producing negative reactions. They are not, however, inherently negative conditions. What is "bad" and what is "good" depends a lot -- some say entirely -- on your mindset.
So the question is, how can you turn a "bad" thing into a "good" thing?