If your mood is off, think about the weather. If it's gloomy, that's obvious - lack of sunshine gets anyone down after a while.
But your mood can also be negatively affected by too much sun - as in, it's hot, or you're stuck inside when you want to be soaking up the rays, or it's too bright and you're head is aching.
Extreme hot or cold isn't good for a Capricorn - it slows progress. But you can do a few simple things to help: get a nice lamp with pleasant lighting, have a sweater on hand and wear layers to adjust throughout the temps of the day.
Get outside for fresh air even if just a few minutes. And when all else fails, blame the weather for your blahs!