The difference between a successful person and others is not a lack of strength, not a lack of knowledge, but rather a lack of will.
I disagree with the last word.
“Persistence despite obstacles” would work for me, but saying they lack “will” is wrong. There is a need for an action in the definition. Success is action, perseverance is action. The definition should communicate the need for action. Many have the will to accomplish something, but do not take action. (On the other hand, some times the obstacles win no matter your action.) But nothing fails faster than a lack of trying.