I hate the word 'depression' because it equates to the unkowing (which is most people thank heavens) with 'depressed' - and they are so very different.
We all get 'depressed' - that is part of the human condition, and is normal, natural, and acceptable.
'Depression' is a whole new arena of experience not related to being 'a bit fed up at all'.
I use this as a comparison - if 'being depressed' is a sprained ankle, then 'depression' us cutting your own foot off with a rusty tin lid.
Theland - try this as a description.
You are a rag doll, and you are in the cage with a large savage gorilla. Some days the gorilla is just 'there' and you are aware of it constantly. Other days it will grab you and throw you around in a blind rage, until it gets bored and throws you into a corner. There you will stay, alone and terrified unitl the gorilla decides tlo have its play again. You can't escape, you can't hide, you are at the mercy of an unthinking violent mass of rage that deals with you when it wants, with no rhyme, reason, or warning. The best you can do is to try to blend in and hope it forgets about you for a while. Medication and counselling, with a large dose of mental strength may open the cage door, but remember, you may find yourself back in there. Never forget that.