might-have-been
noun [ C ] uk/ˈmaɪt.əvˌbiːn/ us/ˈmaɪt.əvˌbiːn/
本来可能发生的事（本来有机会达成的好事，但却没有成真）
something positive that could have been achieved, but was not
There's no point now in regretting the might-have-beens. 对于原本可以实现的事情，现在追悔是毫无意义的。