nostalgic
In this excerpt the speaker is describing how childhood has a magical quality that slips away. He says that the feeling he is currently having is one from his childhood, which indicates that he has not felt it in some time. He says that the energy seemed to never exhaust itself, almost as though it's magic. However, he then states that the feeling or energy slipped away as he grew older. This best fits with the tone of nostalgic. Nostalgia is a longing for something that was once had, but no longer. This is exactly what this excerpt is describing. The speaker has a desire for this magical energetic quality he once had in his childhood but no longer does.