I don't know, hard to say. On one hand its like, duh, he went into the avatar state at the end, so that means he cleared his last "energy pool" and is officially in control of it. We also see BigMe Aang and MiniMe Aang in the cosmos when Azula kills him, so I guess he mastered it. In a sense he didn't let go of Katara though, its wierd. He gave her up because he loved her, you know? He was willing to give up his "love" so he could save his "love". So in??a sense he still loves her because he gave her up to save her. Omg, now my brain is going in circles, its kind of like my theory, that Aang will still be able to love Katara because the avatar state is run by emotions as well as a defense mechonism, love = big time emotions, and what Aang said, "Wait 3 chaukras(?) ago you said love was a good thing!" So, so, so confusing, but there's my take on it.