“If not you, who? If not now, when?” is a great little motivational line that tries to inspire you to personally act.
Diving into the complexities of a task, however, may lead you to respond to the question, “If not you, who” with “not me.” You may not be the best person for the task.
Assessing whether you are, or not, is a question that requires honesty and courage.
If it is the honest answer, you still have an important role to play – how can you catalyze the best person? The thing–whatever it is–still needs to be done.
At some point in the process of getting it done, “you” are the one to play a part.