We usually consider success to be the fulfilment of our desires. Yet when a desire is fulfilled there are more desires. We construe this self-satisfaction to be success and we think there is happiness in it. It is why we pursue it. Will-power as it is understood in this context by a lot of people is the ability of a person to overcome all obstacles to meeting one’s goal, this “goal” usually being sensual satisfaction or an ego-inflating end. So the idea of will-power is based on this success.
Will-power is the strength of the mind (Citta shakti) that makes it capable to meet success or failure with equanimity, composure and clarity. This itself is success because all endeavours of life are uncertain. And we are seeking security and insurance in everything we do, which is why there is no peace in anything we do.
Furthermore, success, understood in the former sense of self-gratification develops arrogance. Failure on the other hand teaches us to be humble and have a clear look at our limitations and helps us to proceed without negligence. This makes us skillful in action, here and now. This helps us to surrender, here and now. This self-surrender and humility are symptoms of happiness.