The moment one definitely commits oneself, the providence moves too.

In searching a misattributed (and mistranslated) quotation, I instead came across two items of note.

The first is from W. H. Murray in The Scottish Himalaya Expedition (1951):

'But when I said that nothing had been done I erred in one important matter. We had definitely committed ourselves and were halfway out of our ruts. We had put down our passage money--booked a sailing to Bombay. This may sound too simple, but is great in consequence. Until one is committed, there is hesitancy, the chance to draw back, always ineffectiveness. Concerning all acts of initiative (and creation), there is one elementary truth the ignorance of which kills countless ideas and splendid plans: that the moment one definitely commits oneself, the providence moves too. A whole stream of events issues from the decision, raising in one's favor all manner of unforeseen incidents, meetings and material assistance, which no man could have dreamt would have come his way. I learned a deep respect for one of Goethe's couplets:

Whatever you can do or dream you can, begin it.
Boldness has genius, power and magic in it!'

I like that parallel with the observer effect in physics, that just as observing changes the observed, Committing changes the context of the commitment.

There is a more raucous translation of what Goethe actually said. From Goethe. The Collected Works. Vol. 2: Faust I & II. Ed. and trans. Stuart Atkins, 1994.

This altercation's gone on long enough,
it's time I saw some action too!
While you are polishing fine phrases
something useful could be going on.
What's the point of harping on the proper mood?
It never comes to him who shilly-shallies.
Since you pretend to be a poet,
make poetry obey your will.
You know that what we need
is a strong drink to gulp down fast,
so set to work and brew it!
What's left undone today, is still not done tomorrow;
to every day there is a use and purpose;
let Resoluteness promptly seize
the forelock of the Possible,
and then, reluctant to let go again,
she's forced to carry on and be productive.

This passage was very loosely translated in 1835 by John Anster as:

Then indecision brings its own delays,
And days are lost lamenting over lost days.
Are you in earnest? Seize this very minute;
What you can do, or dream you can do, begin it;
Boldness has genius, power and magic in it.

This is where Murray got his version of the couplet.