Is a Muslim sinful if he intended to fast Mondays and Thursdays, then limited himself to fasting three days of each month due to slackening?

Fasting Mondays and Thursdays and three days of each month is a recommended Sunnah. If one intends to observe a voluntary fast but breaks it before commencing, there is no blame on him. Similarly, if he begins the fast and then breaks it, there is no sin upon him, and he is not required to make it up, according to the soundest opinion, though it is better to complete it. There is no blame on someone who used to fast Mondays and Thursdays and then switched to fasting three days of each month. However, it is preferable to persevere in obedience, based on the Prophet's (peace be upon him) saying: "O Abdullah, do not be like so-and-so; he used to pray qiyam al-layl but then abandoned qiyam al-layl." And his saying: "Which deed is most beloved to Allah?" He said: "The most regular and constant, even if it is little."