What adjective would you suggest for the like of someone who consistently has a hard time finishing what s/he has started?
I thought of the word "distracted", but I don't feel it's sufficient to describe the full story.
Weak-willed looks appropriate here. Irresolute may also fit. Irresolute is more about changing his mind and not making choices while weak-willed describes literally a lack of will.
"infirm of purpose"
lacking firmness of will or character or purpose
"infirm of purpose; give me the daggers" - Shakespeare
However, this term may also mean lacking a definite objective or purpose altogether.
The colloquial sticktoitiveness is a good antonym for your situation. So perhaps what you need is "xxx lacks any sticktoitiveness."