DEMEA: No one has ever plotted out his life’s plan so successfully that he never faces
New challenges and discoveries brought about by circumstances, the passing of time,
Or experience itself. Inevitably, you discover you don’t know what you thought you did,
And the principles we first formed don’t hold up in the face of everyday practice.

[Nunquam ita quisquam bene subducta ratione ad vitam fuit
Quia res, Ætas usus semper aliquid apportet novi
Aliquid moneat, ut illa, quæ te scire credas, nescias
Et, quæ tibi putaris prima, in experiundo ut repudies.]

Terence (186?-159 BC) African-Roman dramatist [Publius Terentius Afer]

Adelphoe [Adelphoi; Adelphi; The Brothers], Act 5, sc. 4, l. 855ff (160 BC) [tr. Christenson (2010), Sc. 22]
    (Source)

Sometimes attributed to Richard Steele, as he used the quote as an epigraph to an essay in The Spectator, No. 544 (1712-11-24).

(Source (Latin)). Other translations:

No man was ever so completely skilled in the conduct of life, as not to receive new information from age and experience: insomuch that we find ourselves really ignorant of what we thought we understood, and see cause to reject what we fancied our truest interest.
[tr. Steele (1712)]

Never could there be anyone with a method for life so well drawn up, but that circumstance, time, experience ever brings something new, teaches something; so that you know not those things which you think that you know, and that you repudiate in experience the things which you have thought essential for you.
[tr. Giles, c. 1850]

Never was there any person of ever such well-trained habits of life, but that experience, age, and custom are always bringing him something new, or suggesting something; so much so, that what you believe you know you don't know, and what you have fancied of first importance to you, on making trial you reject.
[tr. Riley (1874)]

However well a man may have calculated his scheme of life, still circumstances, years, experience, always introduce a new element and teach new lessons. You find that you don't know what you thought you did know, and what you thought of primary importance that in practice you reject.
[tr. Sargeaunt (Loeb) (1912), Act 6]

You never add things up so well in this life that something unexpected doesn’t turn up and set you wondering -- some money matter, some event, some personal problem. As a result, you find you don’t know what you thought you knew, and things that seemed fine turn out to be worthless when they're put to the test.
[tr. Copley (1967)]

No matter how neatly you keep your accounts in life,
experience, age, and circumstance always show up
to toss something new on the books and teach you a lesson.
You find out you don't know what you thought you did, and what seemed
so important in theory turns out to be pointless in practice.
[tr. Berg/Parker (1999)]

There never was a person so well-bred
And so refined but that into his head
Come new thoughts springing from experience
And age and custom, so that what you sense
You know you don’t know and what you before
Believed was most important you abhor.
[tr. Kelk (2022)]