Proverb: When one door closes, another one opens
This is named Door of Happiness.
“When one door of happiness closes, another opens; but often we look so long at the closed door that we do not see the one which has opened for us.”
– Helen Keller
The earliest reference I have been able to find of this is “This proverbe was fulfild, when one doore is shut the other openeth. (_Lazarillo_, tr. D. Rowland, 1586)”. I was going to choose “All good things must come to an end” but I don’t believe that. I think endings are just transitions into something else. So I remembered one of my favorite quotes and looked it up to see if it was considered a proverb and it’s even listed as a proverb in the 1586 text. Good things don’t come to an end, they just change into something else.