As a matter of syntax purely, both statements are correct, and here's why. Learned/Learnt is both the past tense and past participle of the verb learn. But semantically, i.e. if we consider what you really mean to pass across with that statement, I see why your friend said the second form is the correct one. The first statement "I learned..." seems to narrow your learning to a single simple event, while the second, "I have learned..." sends a message of completed action that might have taken a while. So it seems learning to be yourself(if it actually does require learning) might take some time, which is slightly better depicted by the second statement. Hope this helps.