willpower depletion" might be nothing more than mental distraction by one of these processes

I view it as competion by different processes for a shared resource.

A robust system would have built in mechanisms for time sharing, so that one want doesn't always override the others. Different wants get allocated only so much, and then has to compete again, but at a handicap; some handicap associated with the want, some handicap associated with the justification/payoff for the want.

I'd guess that cycling different stimuli would be more effective than using the same one again and again, and that timing the reinforcement well prior to a "give up" might be effective as well. Keep "topping up" the motivation, instead of letting it run down entirely.