Smile, even if you can’t or don’t want to – be smily. Our minds trick us into thinking we’re happy each time we smile.
Also, laughter yoga exists. Just laugh, even without reason. Eventually, when you’ve done it for long enough, you’ll find a reason to/remember a pleasant memory.
Everything will work out for the good and be alright in the end. If it’s not alright, it’s not the end.
Finally, please remember that it’s a bad day or a bad phase, not a bad life.