I have a feeling everyone has this issue in the beginning. I certainly did. It took me a long time to overcome it too.
In addition to the dry fire with timer, practice live fire with the timer as well. While doing live fire drills, learn to clear your mind of all thought. After the make ready command, and you have loaded and holstered, clear your mind of any and all thought.
At the sound of the buzzer, all those hours of practice and your subconscious mind will carry you through.