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.

Both are correct. The first is past tense; the second (present) perfect tense.

It depends what exactly you wish to say. The first sentence is in simple past, which is used to represents past action and there is no effect at the time of speaking. The second sentence is in present perfect tense, which is also used to say about past action, but in this case there is an effect at the time of speaking. Now, you decide which one is correct. As far as grammar is concerned both seem correct. Hope this could be little helpful to you.