It comes and goes.

Years ago I (semi-)retired from the corporate world for similar reasons and because I thought I would have more fun working on projects related to my hobbies. Turns out that even that got old eventually.

Then a couple of years ago I started working on something else and got renewed passion which continues until today.

Can you take a long break (say 6 months) and go do something completely different (like spend time in the Sahara or something)? When you come back you will have a new passion for your work. Or perhaps you will have discovered a new passion :)

Good luck!