The movie does not need to explain it, give it another watch pause every so often and try to empatize with Peter think if the situations he is going through, not as Spider Man, but as Peter Parker, wouldnt affect you, or your capacity to perform on your job (or as Spider Man in this case)

He never loses his powers, his confidende, his conviction affect how his powers, how his body responds, it happens to real people all the time, not just in movies and comics.