What is the ruling on one who is unable to complete any work they start, whether it be an act of worship or a worldly matter, and always stops halfway through?

Addressing behavioral deficiencies lies in "continuing until the work is completed." This can be aided by dividing the work into small stages and choosing short tasks to begin with, based on the Prophet's (peace be upon him) saying: "O people! Take on only the deeds you are capable of," and his saying: "The most beloved of deeds to Allah Almighty are those that are most consistent, even if they are few." It is also recommended to review books such as "Fatoor: Its Causes and Cure" and "The Inability of the Trustworthy."