I run a checklist in my head whenever im feeling really down. I think about things I know make me really happy, basic things. Friends, family, travel, motorbikes etc. And if when thinking about those I still feel down and pessimistic - I understand that it's my brain chemicals at play and Im in a funk.

Doesn't help me feel better in that particular moment, but does let me know that later it will get better and there some context and comfort in that.

If you are not depressed (big if), and you're just going through the motions of life, you CAN make yourself happier just by reading the right material (e.g., Feeling Good, The Four Agreements) or yes, even affirmations. The happiness will last for about a day, in my experience. Then you can revisit the material.