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What you describe is a 'normal' lowering of mood caused by an external factor - depression (with a little 'd'). We all have things that make us sad - events in our lives, regrets, bereavements etc. You are right that doing something can help take your mind off it, distract you and prevent you becoming obsessed over it.
Clinical Depression is impossible to explain to a person lucky enough never to have suffered from it.
It can be a feeling of despair, worthlessness, paranoia, feeling that there is no point in living because nobody loves/likes you, I feel like I am living in a black hole or tar-like misery and pain - everyone is different.
Suicide seems like it is the only option - an escape from the pain and the thought of feeling like this for the rest of your life is an overwhelmingly awful feeling.
If you know someone who is severely depressed it may be a nice thought to do something 'spontaneously nice' for them. A small gift or treat at a time other than a birthday or Christmas (I have a special atheist Christmas for the cats, Santa Kitty comes to see them!).
I would really like to see more of my brother, I enjoy his company and he has the same, strange, sense of humour as me. He is also very good at household repairs. :-)
But time spent with a good friend can help me escape the misery for a short time. Finding and keeping a friend who is willing and able to cope with you when you are Depressed is not at all easy.
It is now nearing half past six and I better go to bed.