Many scholars and saints have their definitions of humility. Some even have degrees of humility. For example, St. Benedict in his rules has twelve degrees while St. Anselm was said to have seven.
What does humility really entail? For some, it is "a quality by which a person considering his own defects has a lowly opinion of himself and willingly submits himself to God and to others for God's sake. For St. Bernard, it is "a virtue by which a man knowing himself as he truly is, abases himself. For St. Thomas, it is a "virtue” which “consist in keeping oneself within one's own bounds, not reaching out to things above one, but submitting to one's superior".
All the definitions are good and something common runs through. Not thinking high of oneself and submission to higher authority and to God. This makes it impossible for God to be humble since there is no higher authority than him.
However, because he is a mysterious God, he became incarnate of the Virgin and was made man to teach us how to be humble. He asked us to learn of him as man to be meek and humble of heart as a condition for finding rest (Mt. 11:29).
Our heavenly mother adds a new explanation to that of the saints in this apparition. She who is a great model of humility says that in “learning” and following Jesus who is meek and humble of heart, “you must give your all” to become an instrument of God. This is scary because it means crushing the serpent of self with its many heads: self-exaltation, self-seeking, self-sufficiency, self-conceit, self-love, self-satisfaction, self-advancement and self-will (Legion Handbook, p30). May Our Lady help us in this battle that we may conquer ourselves like she did and merit heaven by the grace of God.