(figuratively) to have a strong stomach, to not be easily upset, to have nerves of steel, to have great composure

necessary evil; blessing in disguise

had better watch out, had better brace oneself, to be in for a challenge

he who laughs last laughs best

ill-gotten gains never prosper

(informal) but of course! that's obvious, I can see that now! how did I not get that before?

the better is the enemy of the good; leave well enough alone

not to know how right one is (said when someone who has uttered a statement doesn't know that this statement is even truer than they thought, for example because there is another dimension to it that they weren't aware of)

what comes too late doesn't count any more

charity begins at home