this empty feeling in the middle of your chest, like there's a hole there. An apathetic attitude towards the world. Feeling like life is just a stupid game that shouldn't even be played. Stuff like that.
I don't know which are the first symptoms but I know that some symptoms are a lack of energy, being easily annoyed, lack of interest in things you are normally interested in, a feeling like you just don't care about stuff and feeling like you just want to give up. I hope you feel better soon.
There are two kinds of depression -- clinical and reactive. Clinical depression is the baddie and usually takes therapy and medication to remove. Reactive depression is what you go through when you lose a loved one, or when your world starts falling to bits around you. Fortunately it has a use-by date and you will eventually put it behind you.
Similarly to JD and Retired Kop mine started as a total loss of motivation. I'd just lost my job in a field that was rapidly being replaced by computers. I still had about 15 years left before a reasonable retirement date, and then my wife became very ill. Added to that, I'd recently damaged my knees and could no longer compete in triathlon. It's the only time in my life I've had to endure depression and it took some months to find my feet again.
There are lots of "greeting card" slogans we could quote to help ("this too shall pass" or "on the best of days, on the worst of days, the sun also sets" and so on ad nauseum) but they remain only cliches. With your intelligence and experience you'll get through it. Just be prepared to mark time for a while.
Incidentally, "this too shall pass" is a truism. Ask anybody who's ever had kidney stones.
I want to offer a different perspective so I feel it's best to answer with my daughters who is 27. Basically it's SEVERE hormones where she has just ONE good week a month which is the week before her cycle starts when her estrogen drops off enough for it to start. Then the next 2 weeks she's vrery depressed and very blah, then a week BEFORE she feels great and tries to fit everything in she can possibly do in that week because she feels like she can conquer the world ...then it begins all over again. I'm not saying this is you. I am offering a different perspective which she happened to inherit and she take 5-10mgs of Lexapro which is a tiny amount and became a different person for herself. Her torment seemed to work itself out through all the fluctuations with this tiny dose of medicine and it turned out to be all biological. It still took about a month to set in and work out. She DOES lose that week of where she conquers the world, but the other weeks she feels great instead of depressed. It turned out to be a chemical thing for her. So this is not one of those things people talk about that is PMS-it has cometely different time zones in it.