When J.K. Rowling went through a clinical depression it was the feeling of her illness which brought her the idea of the Dementors. Dementors hunt their prey by sensing emotions. They feed on the positive emotions, happiness and good memories of human beings, forcing them to relive their worst memories.
I think that Rowling did a very good job of visualizing what depression does to you. Depression takes away your ability to see light at the end of the tunnel, to see everything that is going right in your life. You can only focus on what is going wrong and you know deep down that whatever you will try to do next is going to fail. It's a cold and dark place to be in. There is no sun, no warmth, there is only you, alone, failing at everything. I've learned to recognize the symptoms, I know when I am getting depressed. I know that there is little time to fight off the monster that's creeping up on me and I have to act fast. Go out for a walk in the sun, not cancel the things I like to do, get myself of the couch and do the things I have to do and if I feel like I can't do it by myself I have friends and family that will help me. I'm blessed in that regard. But when you are getting depressed, all the people wanting to help you can be like a weight around your neck. It's the people who love you that you have to tell once again that you can't do it. That you are losing the fight once more. That too is a sign of the depression catching up with you, but it's a very hard one to ignore.
The hardest fight though is when depression hides itself in logic. I like logic, I sometimes crave logic. Logic is what makes sense to me, logic makes me feel safe. So when depression hides in logic, I'm in danger. It's my weak spot and it's like my personal Dementor knows this. I've experienced not recognizing the depression once now (apart from my first depression of course) and it's a very scary place to be. Of course when you're in it, you don't realize it's scary.. it all makes sense. It's when you get through it and start to feel better again that you realize that it was touch and go and that even though you are an experience expert at depressions now, you didn't see this one coming and you didn't realize that it was the depression making you think the things you thought.
Most of the time I see my Dementor coming and I'm not scared, I know what to do, I know the dark thoughts are caused by the Dementor, it's not really me thinking them. It's realizing that I'm not always able to see him coming that frightens me.
