Hi
Sorry to hear you are feeling low, if it helps, i mentally plan to move back about every 6 months since iv'e been here - (13 years ) :-0
I've just come back after a month in the UK and i really miss my family and the UK's proximity to everything and everywhere, the humour but mostly just being Home.
Realistically though, how long would that feeling last if i didn't have the option come back , I do wonder .
I'm not big on giving advice, however one suggestion would be to secure your citizenship before you leave , later down the track you may have a change of heart and it will give you options, to return to Australia ( remember it doesn't have to be Adelaide ) or New Zealand. Not sure of your financial position , but perhaps you could make your journey back an adventure, travel a bit maybe in Asia, see something on the way back, it can be done on a budget and it could give you something positive to focus on.
I have a busy life with work and my son but outside of that life is very isolated but when speaking with my UK friends its pretty much the same at our age ( 40's)
Best of luck with your decisions, when one door closes another one opens
Michelle