It’s a bit embarrassing to have been concerned with the human problem all one’s life and find at the end that one has no more to offer by way of advice than “Try to be a little kinder.”

Aldous Huxley (1894-1963) English novelist, essayist and critic

(Attributed)
    (Source)

Quoted in Huston Smith, "Aldous Huxley -- A Tribute," The Psychedelic Review, Vol. 1, No. 3 (1964) (the Aldous Huxley Memorial Issue).

A variant is in Laura Huxley's biography of her husband, This Timeless Moment: A Personal View of Aldous Huxley, "One Never Loves Enough" (1968). She identified it as coming from a "public talk" not long before his death:

It is a little embarrassing that, after forty-five years of research and study, the best advice I can give to people is to be a little kinder to each other.