> Change your job. Change your relationship. Change the city where you live. Change your primary hobby. Change your friends. Go back to school or start a business. Go somewhere you've never been before, the further away the better.

Agreed. It worked for me, at least, when it came to job. I was feeling stuck in my last job. There was nothing to complain about but something didn't feel right. Switching job improved my mood 100%.

Just a fair warning, after initial excitement of new job dies down, you may get buyer's remorse and miss your old job a lot. It was comfortable, you knew what you needed to do, you had built up respect and reputation. Now you will need to start over; just work hard through this period until it passes. Soon excitement and energy will return to you and you will be glad that you switched your job.