I've struggled a lot with depression over the years so I've thought a good bit about this ! I think a big part of happiness which Vic touched on is to be kind, to make a habit of listening to and doing little things to sweeten life for those around us. It doesn't have to cost money. Say something encouraging. Give a long hug.
A very significant part of the happiness I've come to have is in witnessing creation, seeing God's goodness there. Thomas Traherne talks about enjoying the world around us as it is presented to our senses -- the sunlight, the air we breathe, the beauty of colors and shapes in nature: 'If you be not faithful in esteeming these, who shall put into your hands the true Treasures? If you be negligent in prizing these, you will be negligent in prizing all. For there is a disease in him who despiseth present mercies, which till it be cured, he can never be happy. .... Insatiableness is good, but not ingratitude.'
A depression of years began to lift more steadily when I started to list 5-10 things every day for which I was grateful. A mature friend with a debilitating chronic illness expressed to me that even if she were in health -- she would not be happy if she could not be grateful every day, and she can be grateful every day now.
And yes, we should enjoy work whenever we have strength for it. Learn new things, practice something one enjoys, clean the kitchen .... After years of struggling with how to approach work when I often don't have strength, and can't be consistent day to day, I've divided up the things I have to try to keep up with, and those that I am interested in by the energy and focus they demand. Now when I'm exhausted at least I am not scrounging for ideas for some sort of productive engagement (even an audiobook, or a nature video). Work is a gift of God to us, to keep us occupied with gladness in our hearts, Ecclesiastes 5:18-20. And I think anything that requires effort and focus and revision and development is a good discipline in humility, patience, hope, truth -- and love. Our soul gets straightened out a little, even if we don't achieve anything great and lasting.
Maybe it's too theological -- but it is a huge part of 'lifting up our heads': Remember that the handwriting against us has been blotted out. Remember resurrection. The deaths we experience -- even the longest, most drawn out ones -- are not the end of the story. Just the seed that has to fall into the ground.
I am always learning more about happiness in being humbled. I love the song: 'It's a gift to be simple, it's a gift to be free, it's a gift come down where we ought to be. And when we find ourselves in the place just right, 'twill be in the valley of love and delight. ... ' Pride is so miserable -- humility can laugh about things that wound pride because it's already died to them. So at least now when I'm feeling that prideful misery, I *try* to give thanks that God is making me learn what's good for me. In connection with that I would add that happiness is rooted in our experiences as private individuals. The heart knows its own sorrow and a stranger cannot share its joy. We rob ourselves of a huge amount of happiness by living so much in public space, trying to make everything into a recorded, public affair that will be noticed and applauded or commented on. Simply extending ourselves in time and space and notice doesn't bring happiness. The songbird at 3 am that only you hear is likely to bring you more happiness.
Some of these are much harder where there is a chemical issue that needs to be addressed, and feelings may not change even when we do them. And often when people are exhausted they really do just need a break to rest, maybe even a good cry and not to anything expected of them, before their spirits can revive. And there are trials where lament is only appropriate -- God knows. But I do think that small things we do as small people can help a lot to make us generally happier.
