I have aphantasia so one of my techniques for generating happiness while sad might be more effective for you than me... but if I'm feeling really sad, I try to imagine a kitten, riding a puppy, riding a baby goat, riding a baby hippo, riding a baby elephant. Baby animals are always cute and funny, the idea of them all balancing atop each other is ridiculous enough to make me at least crack a smile.

Usually the first step out of sadness or depression is like cracking a door open to reveal a shard of light. We have muscle memory linked to our smile muscles, so whether we feel like smiling or not, finding a way to crack one releases feelgood endorphins.

Then we can take another step towards being happier; exercise, tidying / cleaning our home, going for a walk, getting a task out of the way which we've been avoiding.

If you've never used Audible before, you can sign up for a free trial and keep one book forever; I highly recommend anyone struggling atm use this to get 'Derren Brown - Happy'. The guy is almost literally a mind wizard, the first few chapters explain common self-help fallacies and why they're not effective... iirc chapter 6 / 7 is when he starts introducing a toolkit to increase happiness. It changed my brain though, and I've listened to it several times now. Lots of very useful mental strategies and philosophical tools.