The Psyche of the Shot: Sport Psychology and Competition is a great book to buy and read in English or German that can help improve your shooting performance. If you want to improve your mental game you must read it. The authors (Heinz Reinkemeier & Gaby Buhlmann) are world renowned Olympic champion coaches.
This is my practical summary that I hope can be helpful (it certainly helps me). If you read the book you will learn more. At the end of the book you will also find 13 pages with exercises and mental training programs, including relaxation training, psychophysical regulation, motivation, state of mind, etc.