Yes, winter is so blah sometimes! I keep my curtains/blinds closed usually, turn on some lamps, maybe some cheerful lighting would help? I bought a little humidifier from amazon, it changes colours, very subtle, I only use water in it, but it looks cheerful and you can set it on one colour if you like.
Yes pretending to be happy is hard!! The more you work through this stuff, the better you will feel. I knew it was discussed before, but honestly, there is situational depression and there is help if you go see your doctor, so maybe that's an avenue to explore at this point?