On the practical side, an obstacle that Peter faced as he went through his training to get back to who he used to be was that he was older. He was no longer the youthful Peter Pan, he was the lawyer Peter Banning, and that brought him certain challenges, such as those resulting from the fact that he was out of shape.
On the philosophical side, Peter faced two obstacles that actually went hand in hand: he does not believe that these things are happening to him, and no one else believes that he actually is Peter Pan either. Not only does he think that he is in some elaborate dream when he arrives in Neverland, but he is absolutely adamant that he is a lawyer, not a once-immortal young boy who flew around having fun and playing games all day. This sets him back quite a lot because part of being Peter Pan is that you have to believe; you have to believe in fairies and magic and, most importantly, yourself. Many of the Lost Boys are equally as sure that Peter is not an aged Peter Pan, and they side with Rufio, who claims to be the new Pan. It is not until Peter learns to fly that he believes everything, and when he believes everything, the Lost Boys believe in him, and that is when he becomes Peter Pan.