I used to be...I'm not depressed or anything, but I'm not happy anymore, just indifferent. Your 20s is fun, turning 30 is like getting hit in the face with a baseball bat. I guess I don't have anything to look forward to anymore, the future is just happening. Life has lost all mystery.
Quit your job... go do something else... There you go, something to look forward to which still has some mystery... since it's something new...
No one is truly happy. The best you can hope for are random bits of whimsical, odd, genuine happiness here and there. Life is 97% mundane, hazy slight depression existence worry...and 3% craziness/happiness. The purity of life pretty much ends at the turn of 30. After that mark, you become cynical and jaded and bitter, and each birthday comes closer to death. True happiness...is/was being in love. But not just any love...only young love...when you were in Middle school and High School and College. That magical feeling of thinking about your girlfriend/boyfriend, or imaginary girlfriend/boyfriend. What other moment in life...made you feel like that? A job promotion, graduation, a new car, home, etc etc...None of those even comes close to the magical feeling of being in love. , Love just makes you glow like no other. and gives you energy to move.
In youth we think that happiness will still be, and in old age we think that it was already in our youth ... (c) imho true happiness is in discovery and the realization of one's true potentials its a never-ending journey for people who strive for it
Serious answer, get your testosterone levels checked, both total and free levels. Low testosterone causes exactly what you are describing. Also many people are insecure and lack confidence, that can also cause feelings of depression.
Everyone has their own happiness. I think we don't understand whether we're fully happy or not. But I agree with you about the fact that you forget what happiness is with aging and feel indifference, although it depends on ourselves.