"Just remember, your sole purpose in life is to serve as a warning to others" I think this is from Les Barker.
Brilliant - we all know one or two it fits. Sometimes a look in the mirror on a bad day :-)

I thought it was Piet Hein who wrote:-
"If of thy worldly goods thou art bereft
and of thy slender store, two loaves alone to thee are left
sell one, and with the dole
buy hyacinths to feed thy soul"

Even if he didn't write it, it's lovely and I often think of it in early Spring.

I think he wrote:-
"Here is a thought that might help you fight a bit longer
Things that don't actually kill you outright, make you stronger"