Translation
Paraphrase
What has been bent can never be made truly straight again, which makes life frustrating and full of regrets, and if something (or someone) is no longer with you for whatever reason, you cannot pretend it (or she) is still there.
Footnotes
1: “what is lacking”
If my assumptions are correct, this statement grew out of the pain from having lost his favorite wife in a tragic manner well before old age.