This was the first lesson in my most recent management training, done under the umbrella of the Gustav Kaser Training International (GKTI): ask yourself every morning why are you enjoying yourself (WAIEM).
It might sound like a dumb question, but... think a little bit: if you had at least one reason to be happy that day, wouldn't you feel better? It could be anything: your baby saying the first word, winning the lottery, one award that one of your employees received, or just because it's a late summer day, with some bright warm sun in the sky.
And... if you can't find a reason to enjoy yourself... just make up one! Think about the challenges of that day, and how you plan to attack them, and the results you expect - you might already feel better just because of that!