A lot of good advise here, especially "go see a therapist", but don't underestimate the simple stuff that might get you in this mood. Long periods of bad nutrition, sedentarism and sunlight deprivation have been linked as causes for depression. Look for hacks in your life to help with those while you search for help and answers.
I was going to mention the same. These winter months can be very depressive due to cold weather and lack of sunlight. One of reasons i hate the holidays.
It's important to find sun, travel to a warm sunny destination if you can.
