let's assume you are ambulatory and not confined to a wheelchair. Go outside to the park on a nice day, do some breathing, take in sounds of the birds, trees, wind.

Then remember that lots of people can't walk, lots of people have major issues breathing, lots of people can't see and/or hear, lots of people live in a bombed out warzone w/ no such bucolic parks to walk in. A lot of people are suffering in ways that you are not. When you don't have a job or a girlfriend or friends (certainly a place I've clocked many many years within), this is what "worked" for me. It of course didn't work amazingly but was better than not doing it. The theory is there's always something you can do/experience that you can try to feel good about. People in severe health / war /etc situations will have much more difficulty being happy. But no job/friends/partner, it can be a lot worse. Plus, exercise. I got tons more exercise when i was unemployed and single. nothing but time. I actually cant get good tracts of time to exercise now. grass is always greener etc.