I know there is so much written about depression, but there are so many kinds, and each has its own character. I'm a sufferer from what is called "anxiety depression". What is it like? It's like a phobia about life, a phobia that just won't go away, leaving the mind utterly drained and unable to cope with anything much. (What's the difference between anxiety and phobia? Anxiety results from thoughts about what might happen. Phobia is a gut feeling that bypasses thoughts. Anxieties can sometimes be dealt with by discussion and reasoning, whereas phobias are much stronger and far harder to sort out, and sometimes they never are)
I have had this illness many times throughout my life. It tends to happen at times of stress, but it's not easily predictable. It usually starts with a feeling of fear and vertigo, as if the world is spinning and if I don't concentrate hard I'm going to lose my mind. This is a panic attack. There need be nothing rational or predictable about what starts the attack - once it gets started it can drift from one thing to another, as if my mind is seeking the next thing to worry about, although 'worry' is a huge understatement of the feeling. If the panic attack fades soon, then I'm probably going to be OK, but when it lasts for days, I know I'm in trouble. I end up locked into a state of fear and exhaustion, which soon becomes full depression. I feel everything is grey, that I'm worthless and undeserving, that I'm trapped, normal routine aspects of life become just too much to deal with, leading to feelings of fear and weepiness at the thought of having to cope with them. This can be so sudden that it fits the now somewhat outdated term 'nervous breakdown'. Depressions, for me, don't always start like this - they can start because of other causes of mental exhaustion, but the panic attack is the most common way.
There's no positive side to this kind of depression, it's not like manic depression where there are periods of feeling happy and hyper. It's all down. From past experience, I know what to do to deal with this illness when it recurs. I find a place of security - home, or even just a bed or a sofa, and stay there while constantly distracting my mind with activities such as reading. At the same time, I get from the doctors a prescription of an SSRI antidepressant, which helps calm my emotions and has always worked as a cure before, although it can take a long time (months at least).
Right now I'm on the SSRI, and occasionally able to cope with some work, although I am still having really bad days when I can't manage anything. Feelings of fear and anxiety along with phobia mean I have not been able to travel far for some time, and I can't see that changing as things are - this is agoraphobia and part of this recurrence of depression.
I'm very lucky to have a supportive husband, understanding family, and so many friends I can reach online - past depressive attacks have been very lonely times.
So that's what it's like: fear, anxiety, phobia, lack of mental capacity to deal with what should be routine, low self-worth, exhaustion. It gets better, but it takes a long time.