Yo samski, I relate to your story. I went on a vacation to somewhere I always wanted to go with some of my best friends and the weather was awesome. However, I got really bummed out one night. At the time, my reason was that even though I had what I thought was everything that I could want, I still did not feel all that happy or content. The realization that, even if I could have all of this for the rest of my life, I would still not be satisfied made me feel hopeless. It made me feel like what is even the point of trying if what I was working for wasn't that great anyway. While it may seem like I had no reason to be unhappy, the fact of the matter was that I had a very good reason: I was wasting my life pursuing a dream that was fake. A person's reason for being unhappy (or happy) doesn't necessarily need to be the result of the external environment.
I also relate to the part about how you feel when you are happy, and I think that I psychiatrist would probably describe this as "bipolar disorder" as NoDiety mentioned. However, psychiatry is a very imprecise science, and diagnosis is often very subjective, and treatment is often very experimental. One of my very bright friends who had similar experiences went the route of seeing a psychiatrist and taking medication, and the effects of the changing medication really made his life difficult. I've been through many of these times of turmoil, and while they suck really, really bad, I do feel that the inner conflict has forced me to develop more fully as a person. Like you said, you felt that you were doing anything important. That might be true, and now that you realize this, maybe you can find out what you think is important and try to do that. If you had a completely happy vacation and never realized this, you probably would either never realize this or maybe it would take a mid-life crisis to realize this.
I'm not trying to psychoanalyze you or prescribe the best course of action for you. I am just talking to you as one dude to another. Anyway, I also just recently found an alternative theory to all the disorder theories that I found helpful and can relate to better. It puts a positive spin on the difficult times I was talking about. I just posted about it a little back to see if anyone here could relate. Take it or leave it:
Be Greeted Psychoneurotics.
Suffering, aloneness, self-doubt, sadness, inner conflict; these are our feelings that we have not learned to live with, that we have failed to appreciate, that we reject as destructive and completely negative, but in fact they are symptoms of an expanding consciousness. Dr. Kazimierz Dabrowski has spent 45 years piecing together the complete picture of the growth of the human psyche from primitive integration at birth; the person with potential for development will experience growth as a loosening of the stable psychic structure accompanied by symptoms of psychoneuroses. Reality becomes multileveled, the choices between higher and lower realms of behavior occupy our thought and mark us as human. Dabrowski called this process positive disintegration, he declares that psychoneurosis is not an illness and he insists that development does not come through psychotherapy but that psychotherapy is automatic when the person is conscious of his development.
To Dabrowski, real therapy is autopsychotherapy; it is the self being aware of the self through a long inner investigation; a mapping of the inner environment. There are no techniques to eliminate symptoms because the symptoms constitute the very psychic richness from which grow an increasing awareness of body, mind, humanity and cosmos. Dabrowski gives birth to that process if he can.
Without intense and painful introspection and reflection, development is unlikely. Psychoneurotic symptoms should be embraced and transformed into anxieties about human problems of an ever higher order. If psychoneuroses continue to be classified as mental illness, then perhaps it is a sickness better than health.
"Without passing through very difficult experiences and even something like psychoneurosis and neurosis we cannot understand human beings and we cannot realize our multidimensional and multilevel development toward higher and higher levels." Dabrowski.