I thought there was another word you needed to use to change the sentence from "I was" to "I used to be".
I think the sentence is fine translated this way, and could also be understood as "I was an introverted lad". However I suspect that you are thinking of the habitual action:
O'n i'n arfer mynd i'r ysgol : I used to go to school.
Arfer can be used to explicitly form the habitual past (or other habitual tenses) where the action was habitual. I used to go to school. It was a habit, something I did routinely but now I don't. It is not the only way to express that, but is probably more common for habitual action.
I think it would sound strange to say the boy was habitually introverted but isn't now. You could say that of course, but here the sense of the sentence was more "I was an introverted lad (but now I am not)" In English we can express that as "I used to be an introverted lad".
If I was answering this sentence I would translate it as "I was an introverted boy".