I woke up in the middle of the night, the night before tisha b'av feeling sick. In my head this passuk kept repeating itself. I got up to learn a little and try and put myself to rest, because when you wake up in the middle of the night it is good to learn something in case you awoke for that very reason.
I started looking up perushim on the passuk but before I found anything, it had switched in my head to סלח נא ביד תסלח - forgive, please, by the hand of the one you will forgive.
In any case, I kept looking, Rashi, Ohr HaHayyim haKadosh, Ba'al Shem Tov, Heichal Berachah, I didn't find anything that explained my dilemma to me, or gave me any peace of mind. In the end I opened Pri Ha'aretz (Reb Menachem Mendel of Vitebsk, the first Hassidic Rebbe to make it to Israel and write his perush on Torah here) and sought out parashath Shemoth. It seemed to have only a short paragraph on an earlier passuk in Shemoth, but once I looked into it, it did indeed mention the passuk in question--how could it not?--but I didn't get to finish reading the paragraph, instead I sneezed and the book flipped to Parashath Vayeshev. I looked down and there was a Torah on the holy hebrew letters, a topic dear to my heart and something I am currently learning in the Notzer Hesed so I kept reading, he is explaining how Tzaddikim rise up throught their tefilloth above the space of letters and bring about all the needs of Israel, on the second page we get to what I was looking for:
It is only through smallness, through lowliness that we can accomplish two things: 1. To raise ourselves up from lowliness to greatness, and more important 2. That we can raise the Shechinah up from lowliness back to Her rightful place of greatness.
Chazal explain that a person who wants to remain great always and never wants to fall is a fool, because he thinks he has intrinsic value to the world--man's only value is that in raising himself up again, he raises others (including the Shechinah) up with himself.
This he says, is the tikkun of the sin of Adam HaRishon. Both Yaakov Avinu and Mosheh Rabbeinu had the face of Adam HaRishon because it was there role to perform this Tikkun of Adam HaRishon. (This, he explains, is why Yaakov Avinu was punished for attempting to sit in peace rather than fighting the battle against the klippoth.)
Here, everything becomes clear. The Ohr HaHayyim explains that Mosheh tells HaShem בי אדני - the flaw is within me, I'm not a vessel fit to carry out Your will. Now, we can see from the Pri Ha'aretz why this was such a problem. The responsibility fell upon Mosheh to recognize that his flaw was precisely what made him worthy to perform such a mission. He could raise himself up and through this he could raise the Shechinah and all of Am Yisrael up with him. For someone lacking this flaw, like Aharon HaKohen, this could not have been possible.
So it is fitting that we read Mosheh Rabbeinu's words in this new way: סלח נא ביד תסלח - forgive please, by the hand of whom you will forgive. Since only someone who is lowly is in need of forgiveness, and HaShem forgives all those who turn to Him in Teshuvah. Let us ask HaShem to forgive all of klal Yisrael because it is appropriate that we ask it of Him, because only someone who has fallen has the power to rise up and raise up the Shechinah and the klal along with him.
It is our flaws that give us the leg to stand on, that we might be able to ask forgiveness not only for ourselves but for everyone and everything.
It is precisely because we are so flawed and so low and have so much left to do that we can bring about the geulah.