I think you have to decide to have a good day.
I mean, not in a way where you put pressure on yourself, like it's another thing to check off your already impossible To Do List. You shouldn't be going around with the cords on your neck showing and shouting to yourself through clenched teeth DAMMIT! I WILL HAVE A GOOD DAY OR ELSE!
But you can float the idea out there to the Universe. Gently. As in, "Wouldn't it be nice if instead of letting all this crap I'm contending with get me down, I just decided to--you know--stay upbeat. Be of good cheer."
Along the way you can also remind yourself not to give other people the responsibility to make YOU happy. Do it for yourself. Watch the hummingbirds in the back yard. Pet the cat. Eat a cupcake. Buy a cheap pair of earrings at Target. Read a mystery. Cut some roses and put them in a Mason jar. You know. Stuff like that.
I'll tell you if it works, okay?