Weakness of will

Hello Rabbi.
I recently heard a conversation by the rabbi in which he casually mentioned two options for understanding weakness of will: Option A – The very fact that a person wants A and does B indicates that his will is not really where he thinks it is, or rather, he has additional desires (for example, lust) that are sometimes more disciplinary than the will he attributes to himself.
Option B – A person truly wants a certain thing but does it because of weakness – weakness of will (weakness in execution?)
Personally I don’t feel that the first option is the right one although I don’t understand the second option! From a purely logical point of view if I don’t do A even though it’s only up to me I probably don’t really want it wholeheartedly! What goes wrong in the process?
I would be grateful if the rabbi could explain to me (or refer to an accessible source if the rabbi does not have time) the mental and conceptual logic of the second option and the way the choice works according to this option? What is the cause if not a will against me?
I hope I was clear, thank you and good night.