Sad thoughts sometimes brought on by seasonal depression easily overcome us if we are not active.
Your physiology creates your psychology which means how you treat your body is how you will feel or let you deal with problems more effectively.
We only have a certain amount of "mojo" each day, and if we wear it out mulling over things and making ourselves more miserable instead of taking positive steps, then you will find yourself in a downward spiral.
How to snap out of it? Write down the problems that bother you most, then cross out the ones you currently cannot do anything about and focus on the ones you can take positive steps to complete, fix or work toward.
Write down some medium and long term goals of where you want to be in 3 months, 6 months, 1 year.
Work toward the goals.
And treat your body well.
Don't skimp on healthy food, eat some fruits and vegetables and get regular exercise even if you just force yourself to take a 1/2 hour walk each day (ipods are allowed on walks!).
Once your body starts to feel better (cut back on junk food, sodas: anything with caffeine in it, treat your temple well and it will return the favor).
Take one step at a time and reward yourself! Life is too short to be making yourself sad.
Fire the little "sadmaker" in your head, straighten up your posture, take deep breaths and get active, focus on a more positive outcome and good things will happen for you.