Psychology, what goes on inside the mind while playing, is extremely important.  As the trumpet player improves, the body learns to react to the thoughts in the mind.  What comes out the bell is a direct reflection of what goes on in the mind.

Several books deal with this topic extensively.  The Inner Game of Music by Barry Green is written by a musician for musicians.  It requires very little effort to apply.  W. Timothy Gallwey  is the originator of inner games of the mind principles.  His first book, The Inner Game of Tennis, is excellent.  Every time it refers to “tennis,” substitute “trumpet playing.”  It is very easy to apply his writing to trumpet playing.

I’m OK, You’re OK by Thomas A. Harris and Psycho-Cybernetics by Maxwell Maltz are also valuable books, but they require more mental effort to apply to trumpet playing.  They also should be read, but after the Inner Game books.

Unlimited Power: the new science of personal achievement by Tony Robbins is extremely thorough and very helpful.