Hi, Amanda_1981! Sorry to hear you suffer from Winter Poisoning.
Some tentative ideas drawn from my experience - your mileage may vary:

• Recognize and accept you are in a depressive low. (To be fair, alternate approaches exist. Some people advocate not accepting your state and using positive thinking or positive affirmations to energize oneself against the depression. That approach has not worked for me; it does work for others.)
• Adjust your activities accordingly to accommodate your condition. This is analogous to accommodating a flu or broken limb.
• Pamper yourself - whatever "pampering" means to you.
• Avoid making significant decisions; put them off till the worst of the depressive episode passes.
• Avoid or put off triggering encounters/tasks/situations.
• Do only what is necessary; conserve your psychic energies.
• Do not attempt to maximize or impose efficiency on whatever activities you are able to undertake (I personally find trying to be "efficient" while significantly depressed actually undermines my feeble attempts to be minimally productive).
• Get as much refreshing sleep as much as you can.
• Etc.

My experience with a really expensive "light box" was less than helpful. Light therapy does help many people, though. Look up "full spectrum lighting" for possibly less-expensive options.

Best wishes for dealing with winter!