Or HaChaim on Exodus 4:10/אור החיים על שמות ד׳:י׳

1 ויאמר משה בי א־ד־ו־נ־י לא איש דברים אנכי, Moses said: "Please my Lord, I am not a man of words, etc." After Moses' previous arguments had been silenced, Moses now appealed to the attribute of Mercy. This is why the Torah emphasises the name of G'd Moses used here. Moses had not used that name of G'd previously. The word בי is a word of entreaty.

2Alternatively, we can understand the word בי as a pronoun in which case Moses would have answered G'd that he considered his own unsuitability for the mission as due to בי, his own shortcomings. As soon as the people would become aware of him stammering they would realise that even after he pronounced the words he had no command of language. They would reject him. Moses added the words גם מתמול גם משלשום, to point out that despite his repeated conversations with G'd, G'd had not cured him of his deformity. It appeared therefore that he was meant to suffer from this deformity also in the future. If so, the people would surely reject him because they would think that if G'd had spoken to him He would first have cured him of that blemish. (He left the last part of the sentence unspoken).