For always the man in whom thought springs up over thought sets his mark farther off, for the one thought saps the force of the other.

[Ché sempre l’omo in cui pensier rampolla
sovra pensier, da sé dilunga il segno,
perché la foga l’un de l’altro insolla]

Dante Alighieri (1265-1321) Italian poet

The Divine Comedy [Divina Commedia], Book 2 “Purgatorio,” Canto 5, l. 16ff (5.16-18) (1314) [tr. Sinclair (1939)]
    (Source)

Virgil telling Dante he's overthinking things, letting himself be distracted.

(Source (Italian)). Alternate translations:

He, that permits his Fancy thus to stray.
With every lure, will rarely find his way
To that great end, to which his soul is bent:
For gath'ring fancies warp the steady light
Of Reason's beam, and leave her whelm'd in night,
For ever baffled of her first intent.
[tr. Boyd (1802), st. 3]

He, in whose bosom thought on thought shoots out,
Still of his aim is wide, in that the one
Sicklies and wastes to nought the other’s strength.
[tr. Cary (1814)]

He in whose bosom thought springs up to thought,
Destroys himself the figures of his loom --
The birth of one prepares the others's tomb.
[tr. Bannerman (1850)]

For evermore the man in whom is springing
Thought upon thought, removes from him the mark,
Because the force of one the other weakens.
[tr. Longfellow (1867)]

For ever the man, in whom thought wells up over thought, removes far from himself his mark, because the rush of the second slackens the first.
[tr. Butler (1885)]

Always the man in whom new thought doth grow
On previous thought, from his true course doth roam,
Because the one doth flag the other's glow.
[tr. Minchin (1885)]

For always the man in whom thought on thought wells up removes from himself his aim, for the force of one weakens the other.
[tr. Norton (1892)]

For ever the man in whom thought wells up on thought, sets back his mark, because the one saps the force of the other.
[tr. Okey (1901)]

For always he in whom thought overtakes
The former thought, his goal less clearly sees.
Because the one the other must relax.
[tr. Binyon (1943)]

He aims beside the mark whose fancies bubble
One on another, driving back and drumming
Each other out, so that his eye sees double.
[tr. Sayers (1955)]

For when a man lets his attention range
toward every wisp, he loses true direction,
sapping his mind's force with continual change.
[tr. Ciardi (1961)]

For always the man in whom thought wells
up on thought sets back his mark,
for one thought weakens the force of the other.
[tr. Singleton (1973)]

The man who lets his thoughts be turned aside
by one thing or another, will lose sight
of his true goal, his mind sapped of its strength.
[tr. Musa (1981)]

Because the man in whom thoughts bubble up
One after the other, goes wide of the mark,
Because one thought weakens the force of another.
[tr. Sisson (1981)]

The man in whom thought thrusts ahead of thought
allows the goal he’s set to move far off --
the force of one thought saps the other’s force.
[tr. Mandelbaum (1982)]

For always the man in whom one care sprouts above the other makes his target more distant, because the impulse of the one weakens the other.
[tr. Durling (2003)]

Since the man, in whom thought rises on thought, sets himself back, because the force of the one weakens the other.
[tr. Kline (2002)]

When thought is bred too rampantly from thought,
then, of himself, a man will miss the mark.
Each mental thrust debilitates the first.
[tr. Kirkpatrick (2007)]

For any man who lets one thought --
and then another -- take him over
will soon lose track of his first goal.
[tr. Hollander/Hollander (2007)]

A man whose mind is distracted lets thought after thought
Keep him from getting where he wants to go:
They hammer each other down; nothing can grow.
[tr. Raffel (2010)]