I've been watching that trailer over and over trying to see if I could get anything else from the cosmos Aang...thing, and I can't. It's like jumping into the middle of Stephen King book, makes no sense.
But, and I'm really sorry to for starting this over again...when the Guru is talking to Aang, he says, "If you leave now you won't be able to go into the Avatar State at all." He stresses the "at all" part.
Maybe it's wishful thinking, but it sounds to me like Aang made his choice and the Guru is telling him that if he leaves immediately, not only will he not know how to control the avatar state, but he can't at all.
Does that make sense?