Henry James, The Middle Years

“He was tired enough when he reached it, and for a moment he was disappointed; he was better, of course, but better, after all, than what? He should never again, as at one or two great moments of the past, be better than himself.”

Henry James, The Middle Years, (Henry James: Complete Stories 1892-1898, 335-55).

Why is he, Dencombe, tired and disappointed? Was he injured in some way and why does he believe he will never be the better version of himself in the past?