db,
You mention your sister, who's a physician, who has worked with people who are clinically depressed. Yes, they literally can't function. You say you do function and sleep and have an appetite, but you are joyless, hopeless and lonely. This is not a strict "clinical depression" but a low-grade form of depression, dubbed "dysthmia." Chemicals don't cure this, and besides, you've tried them and they don't work; longterm psychotherapy makes it manageable and can ultimately "cure" it. ALL of us suffer from depression, as you must know. You need help right now. Don't go it alone.
You say it's a "spiritual thing" with you and I believe you. Meds will only slightly alleviate the issues, like a painkiller that doesn't cure the underlying cause.
You sound lost spiritually. There are things you can do about that. Concrete things. Personally, I found Christianity and the other major religious systems to be unhelpful. Then, I explored Zen Buddhism. Its particular form of meditation enabled me to isolate my thoughts of crippling despair and move on. I haven't looked back.
"Life," as Neitzche (I think) said, is "brutish and short." You must rise above all that and your own "ego," or sense of self, that is dictating to you that you must DO THIS or BE THAT to be a worthy person.
There's only two things to do on this earth: first, do no harm, secondly, watch over and care for those you love.
Be of service and get out of yourself. Help the poor, the ill. If you won't commit to long-term psychotherapy, then you must learn to stop thinking of yourself and your woes in order to move on.
You mention seeing MDs who prescribe meds, which you hate. But, you haven't mentioned building a long-term relationship with a counsellor or a psychotherapist who will slowly, gently help you uncover the sources of your discontent and then help you learn how to alleviate them.
It takes a lifetime to grow up where your life becomes bearable and full of contentment. Forget "happiness": it arises unbidden and usually as the result of doing something kind for someone else.
Give to others if you want to heal yourself.
Once again, I wish you the best.
Whit