I went through a period of about a year where I went through this after quitting drinking, it was like everything was infuriatingly boring without alcohol to make me have fun. You didn't mention substance abuse being an issue so this may not be the trick for you, but what helped me snap out of it was finding a challenging physical activity that you enjoy doing (might take some experimenting) such as a martial arts practice, archery, fencing - whatever interests you. What did the trick for me was a blend of yoga and pole fitness (pole dancing). I was never a very athletic person and sports didn't interest me, but finding an activity that put me out of my brain and into my body that turned out to be really fun completely turned my attitude around. Good luck!