We change our minds significantly less often than we think. Generally, if you can predict what your decision will be – even if you, consciously, don’t think you’re all that certain – you’ll be right about 95% of the time.
This is particularly relevant when trying to make yourself do something. No matter what arguments you come up with or appeals to duty and consequences you use to cudgel yourself out of bed, say, if you pay attention you’ll know what you’ll end up doing before you even start.
For this reason, the idea of “willpower” as a magic force that you can use to just change your decision to the “right” one at the last minute is patently ludicrous. Rather like trying to tell a child to “think before you act” – if you’re the kind of person / in the kind of mood to “apply your willpower,” you’re also in the mood to just do whatever-it-is anyway. The application of willpower is a proxy, a way of implementing a decision already made – we control much less of our brains than we’d like.
So… there’s really, spectacularly no point in telling someone “Just Try Harder!” Go figure out how to put them in a willpower-using mood, instead.