can't find a single instance where there is a citation
It's from "Lancelot and Elaine" – chapter 7 of Tennyson's "The Idylls of the King" (1859)
The story is that Elaine falls in love with Sir Lancelot, who doesn't love her. She dies of a broken heart, and the line you used is in the letter she wrote, saying why she had died.