מִזְמ֥וֹר לְדָוִ֑ד יְהוָ֥ה רֹ֝עִ֗י לֹ֣א אֶחְסָֽר׃
אַ מזמור פֿון דָוִדן. גאָט איז מײַן פּאַסטוך, מיר װעט ניט פֿעלן.
Mei HaShiloach
This means that Yaakov made himself smaller in comparison to his forefathers Abraham and Isaac, and said that they had greater wisdom and understanding, until they widened the (underlying) will of G-d from their own abilities. Even though G-d opened up a small opening (that enabled them to do so) they widened it with their wisdom in order to do the will of their creator, and understood things that G-d didn't explicitly reveal to them. This is the meaning of the words "HITHALCHU" - they walked from their own power, but I (Jacob am different) because 'G-d is my shepherd until this day' -i.e. that in every detail of every thing that I do, I need G-d to enlighten me as to his will. Even in this situation where I have seen G-d's will (of my own accord), I still need to see at this time what G-d wants of me. In this light, Jacob is the greatest of the forefathers, because it is a higher level for G-d to constantly be leading him. This was the prayer of David of blessed memory, when he prayed: " G-d is my shepherd I shall not lack", meaning that I should never lack the protection that G-d has over me, and that he should constantly shepherd me, and I should constantly recognise that G-d is my shepherd. Because, G-d, in his goodness leads everything. Even if man turns his head away and doesn't want to expect (something to occur) or to pray, and at a time when a person prays a complete prayer then he is answered, and he will see that G-d is the shepherd. This is the meaning of the words " I will not lack" - that I will not turn my face away from him.