The best thing about this thread is the realisation that it can happen to anyone and you are not alone. I've been following it quietly for a while... The black dog has followed me around for the last 15 years. The birth of my first child three years ago brought everything to a head and as I wasn't coping (having a child made me feel very lonely, it seems odd, but I know other people who have felt the same) I was forced into getting professional help as much for the sake of my family as for me and because I was close to ending it all. It did help dramatically. It helped me understand better what was going on in my head and how to recognise even the earliest signs of a dip. I'm quite scientific and analytical and so an analytical approach to my depression helps me drag myself out of it, eventually. Although this same analysis can sometimes lead me to over analyse things in my life and causes me to become depressed. I can't win!
However, for me, the most powerful thing that has often dragged me back up was a friend telling me," instead of thinking I must or I have to or I can't do, think, I get to." So I get to go to work each day, I get to have my wife, my children, I get to walk my dog etc. It seemed a cheesy thing to say at first, but at the first sign of me slipping it has dragged me up a good number of times. I have a stressful job ( don't we all) but now rather than wake up and think I can't face going into work, I often find myself saying at least I get to go to work. You get the idea. Give it a go, but I can't recommend professional help highly enough. It's good to talk to someone who won't judge you, won't become angry with you because they are not emotionally attached to you and because they understand what is actually going on in the mind. I'm a lot lot better now. Not perfect, but I would call myself generally happy. There is light at the end of the tunnel. Good luck.