The more you learn, the better you perform, and do so with reduced fear. Insist that your training be as realistic as safety permits. If a defensive tactics technique looks unrealistic to you, politely ask the instructor about it. If a shooting technique lacks real-world practicality, speak up. The more realistic and practical your training, the more confident you are.
Mental imagery is sweatless practice.
You can do it in a chair or in your patrol vehicle. In your chair, you can conjure a family fight, an unwanted bar drunk or a mental screaming on a street corner. While you sit there all warm and comfy, see yourself handle the situation smoothly and confidently. When in your patrol vehicle, park near a mini market, bank or liquor store and visualize how you would respond to a hot robbery in progress. See your actions clearly in your mind and with as many variables as you can create.