"The grass is always greener", but about "good old days"
Those were the days!
TFD(idioms):
Those were the days!
something that you say which means life was better at the time in the past that you are talking about
We had no money but we were young and madly in love. Oh, those were the days!Cambridge Idioms Dictionary, 2nd ed. Copyright © Cambridge University Press 2006. Reproduced with permission.
To quote a long ago commentator, nostalgia ain't what it used to be.