Idiom: to hesitate when something is nearly finished

Solution 1:

You're very close:

to falter at the final hurdle

Reasonably recent examples of the metaphor include:

Liverpool restaurant owner determined not to "falter at the final hurdle" after 'tough' lockdown

[Elle May Rice; MSN]

Rugby: Ponty falter at the final hurdle: DOUBLE dreams died for Pontypridd against a dominant Llanelli

[WalesOnline]

Solution 2:

To get cold feet

Commonly used for doubts before marriage, but also used for hesitation in completing a task. It has an interesting history, as discussed here: Why do we get cold feet?

Solution 3:

To have a (sudden) change of heart might be the phrase you're looking for.

It means to have a sudden reversal of one's feelings, intentions, opinions, etc.

E.g. He had planned on retiring and, at the last moment, he had a change of heart.

Or, you might do also with:

To have second thoughts e.g. He had planned on retiring but, at the last moment, he had second thoughts.

And

To waver in the homestretch.

Solution 4:

To opt out/ pull out/ withdraw at the last minute OR at the eleventh hour

Solution 5:

Procrastination covers any first, second, last or any thoughts that you might have.