Reorienting Thoughts Towards Virtuous Object I noticed that if I set an intention to avoid some behaviour, I rarely follow through. I feel perhaps a mere intention based on willpower doesn't have much strength. 

Is it more effective to direct one's thoughts to a virtuous object whenever fantasies and cravings, or worries and rumination, occur? Are these cravings/worries causing the problems?

Is just setting an intention mentally sometimes not adequate?
You might want to consult Vitakkasanthana Sutta: The Removal of Distracting Thoughts
https://www.accesstoinsight.org/tipitaka/mn/mn.020.soma.html

It explains how one deals with unwholesome thoughts in several ways and 'Reorienting Thoughts Towards Virtuous Object' is one of those ways.