Looking for a midrash about Moshe (preferably in English) that says that Moshe rabenu was not born perfect but over the early part of his life perfected his character...
This would be puzzling because
the Gemara (Sotah 12) states, “’She saw him, that he was good…’ R. Nechemiah says, ‘Fitting for prophecy.’ Others say, ‘He was born circumcised.’ And the Sages say, ‘At the time Moshe was born, the whole house filled with light. [...]
When Pharoah’s daughter found him it says: ‘And she saw him, the boy.’ It should have just said, ‘She saw him.’ Explained R. Yosi bar Chanina, ‘Because she saw the Shechinah with him.’”
but strangeside has a relevant story referring to your question
Over a century ago, in 5655/1895, Rav Chaim Yitzchok Aharon, the Maggid of Vilkomir, published an eleven-page booklet, “Klil Tiferes,” to explode what seemed to be an impossible story. “In this brochure,” Rav Chaim Yitzchok Aharon writes, “we have come to point out that the Torah testifies that Moshe Rabbeinu possessed every good virtue at birth, as it says, ‘And she saw him, that he was good…’ (Shemos 2:2).
“However, about fifty years ago, one of the gedolei hador, (the “Tiferes Yisroel”) cited an anonymous story in his sefer, at the end of Seder Nashim. The story testifies that Moshe was inclined, by nature, to have bad middos, such as pride and avarice… The gaonim of our generation agree that this letter must have originated in some journal… Therefore, I have written this brochure to prove that this story is a total forgery and absolute falsehood…”
see there for the full details, too long to quote here