“Love is when the other person's happiness is more important than your own.”

John Tyree, Epilogue, p. 275
Source: 2000s, Dear John (2006)

"Jubal Harshaw" in the first edition (1961); the later 1991 "Uncut" edition didn't have this line, because it was one Heinlein had added when he went through and trimmed the originally submitted manuscript on which the "Uncut" edition is based. Heinlein also later used a variant of this in The Cat Who Walks Through Walls where he has Xia quote Harshaw: "Dr. Harshaw says that 'the word "love" designates a subjective condition in which the welfare and happiness of another person are essential to one's own happiness.'"
Source: Stranger in a Strange Land (1961; 1991)

Letter to Isham Reavis (5 November 1855)
1850s
Context: If you are resolutely determined to make a lawyer of yourself, the thing is more than half done already. It is but a small matter whether you read with anyone or not. I did not read with anyone. Get the books, and read and study them till you understand them in their principal features; and that is the main thing. It is of no consequence to be in a large town while you are reading. I read at New Salem, which never had three hundred people living in it. The books, and your capacity for understanding them, are just the same in all places.... Always bear in mind that your own resolution to succeed, is more important than any other one thing.

After the Theatre (1892)

Actually said by Charles de Gaulle, on leaving his presidency, Life, May 9, 1969.
Misattributed

Attributed to de Gaulle by Romain Gary, Life, May 9, 1969
Fifth Republic and other post-WW2

Actually said by Charles de Gaulle, on leaving his presidency, as quoted inLife' (9 May 1969)
Misattributed