Be warned, folks! Depression is not about sadness, but about not being able to do things.
If you are feeling incapable of doing things, like physically siting in front of the computer and not being able to do any work, you are probably depressed.
Sometimes you'll sit in front of the computer, try to start some task but somehow end opening hackeer news, digg, youtube.
Seek help as soon as possible, do not wait until you are fired.
I have to strongly disagree.
If that kind of behavior gets out of control it sounds like typical adhd. One still wants to do stuff but unable to get oneself into actually doing it.
Depression IMO is when one starts losing motivation and ability to derive pleasure from activities that were normally pleasant.
'Until you make the unconscious conscious, it will direct your life and you will call it fate.'
It looks to me also that there's a bit of idealization going on on your side. In any case you'll work with other people and I bet you won't feel too good if you'll be doing env. work with total assholes. So I would also look at that angle -- being among people you like/appreciate and who reciprocate also kind of eases that existential thing (IMO).
Now I am working in the field and organization which definitely are worthwhile / good cause (imo). But it comes at a price, I wouldn't mind to work in a less meaningful place (subjectively) for a few years with a significantly higher comp.
Also good cause is a motivator but the not the most immediate one. Used to work in a slightly dysfunctional company but my responsibilities were (mostly) so well defined and managed day-to-day that just getting stuff done was satisfying.