חושן משפט - הלכות אונאה - הלכה ג - אות א
אהיה אשר אהיה - I will be who I will be (שמות ג:יד) – This posuk corresponds to the idea of doing teshuva on a previous teshuva, because teshuva is an aspect of אהיה, I will be. This is an aspect of “I am prepared to be”, because before a person does teshuva it’s as if he never existed in the world at all. And, when one starts to do teshuva he becomes as aspect of “I will be”, because when he starts to prepare himself to have a real existence in the world he is thus an aspect of “I am prepared to be.” But, even after he does teshuva, and he is worthy of אהיה, for he has prepared himself to have an existence in this world, he nonetheless must do teshuva on that teshuva. And, therefore he must start every moment anew, and even though he already did teshuva he must afterward start anew completely.
Reb Nosson is elaborating on a tremendous yesod in life. One’s very existence is not determined by what he can do. Hashem gives everything thing that exists worlds of potential, so what a person can do is of no significance. However, the degree to which a person exerts himself and uses his potential to bring kedusha into this world is the real barometer of one’s existential value.
It’s integral to one’s growth that he constantly strive to better himself. One must do teshuva on a daily basis, because even though he may have completely negated his yezter in one particular moment, there is an infinite amount of space to grow. He must ask himself what he can do now and what more is there to be done. And, if he fails ר"ל, then how much more integral is his teshuva? It's not the past that matters. The past already happened and is gone forever. What matters is what will be.
We must realize that stagnation is a negation of existence. The very definintion of our existence is that we are constantly growing and moving. Why would Hashem have put us into this world if He wanted us to remain static? "I will be who I will be!" Hashem’s statement to Moshe is in the future tense, and is a testament to the necessity for us to constantly grow and expand on our current existence, because our lives and the imminent geula of Klal Yisrael depends on what is to be only seconds from now.