"Is there a definitive dividing line between these two? "
Not a simple question Tilly and that definitive dividing line that we are all talking about, varies from person to person.....even if it exists.
Depression.......is not just one disorder and there are various types of depression, but reading you OP I guess, just guess, that it is reactive or exogenous depression.......something that has sparked it off.
My definition.....if you mental state is affecting your everyday life...sleeping pattern, eating, day to day activities and you cannot be bothered to get out of bed in the morning.......then you are clinically depressed. But......need that be treated?
Only a Dr experienced in mental disorders could advice.
I have often been fed up.........but never to the extent that i contemplated seeking medical help.
So, I feel that I have never suffered from depression........it is a relative state of mind.
I am no expert on Psychiatric disorders.