I... don't really get the lesson, personally. Or when she supposedly got it.
So, a long-haired person grabs her by it and talks to her about how softer than her own it is while she is weaker, so she probably spent more time caring for it than training. So she cuts her hair out to escape and never grows it again. Later gives the lecture she got to Ino herself, trying to show herself as more mature.
Later, three of the strongest living ninjas in Konoha have long hair, Tsunade's two braids being the shorter of the sannin. Tsunade is also her teacher, and she supposedly became better than her.
I really think she took Kin's comment about caring more for her hair than for training too literally and liking the short-hair result. Really, the other excuses for short hair were fan-made as far as I know.