"Share a little Christmas cheer"
There is joy is forgetting about one's own troubles and taking note of what other people around you might be going through. There's a very good chance that when you look around, there will be someone who could do with a little Christmas cheer, maybe even more than you do.
Be a Santa's Elf ... or a Christmas Angel ... and do something for someone else. If you live in an apartment, hang a little treat bag on their door knob. Fill it with cookies or Christmas tea or a cute little felt snowman. A little book. A candy cane. Hot chocolate. Leave a note that says Merry Christmas from a Secret Friend.
And, of course, there are the times when someone may ask you for loose change. Bundle up an envelope ahead of time with a few spare dollars and maybe stick in some chocolate. Keep a spare knit hat or some socks in your backpack. Be prepared for opportunities. Make it into a little adventure. When you start looking for someone else who could do with a little Christmas cheer, suddenly you're experiencing the return of your joy.