I like to think of depression as a medical condition where, despite what's going on in your life, despite how you think, you will always be depressed. However, if you are always going to be in an untenable situation, then you will be deeply unhappy, unfulfilled, etc etc. For all intents and purposes, if you can't solve what's making you unhappy and it's an ongoing problem, you're also going to be depressed.
I would love for there to be a terminology difference between these 2 things, but I really don't think there is. Where depression comes from isn't necessary as part of the diagnosis of depression, and I suppose that makes sense because we know jack and s**t about what causes it. Take your vitamins, exercise, and eat right is about the best help out there as far as I'm concerned.
If you call yourself fed up implies that you're fed up about something, which implies to me that it's not due to your emotions not working properly. Of course the vast majority of people who are depressed due to something being broken in their brain or body or whatever, will have crap going on in their lives, like most people do, and so it can be hard to really know what's truly wrong with you.
I agree that if there's a light at the end of the tunnel, it's not depression. The depression tunnel's too long for that!
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Not autistic, I think
Prone to depression
Have celiac disease
Poor motivation