So instead of stressing about things out of your control, embrace the unknowns!
Every athlete makes mistakes on race day. Struggles are not limited to age group athletes. All athletes, of all levels, experience battles with the mind and body in almost every race.
Rather than going into workouts focusing only on the metrics, focus on your execution and how your body responds to given efforts. Complaining over a bad training session doesn't prepare you better for your race. It only causes you to question your abilities. Don't take the easy way out and just call it a day every time when you feel like your workout isn't going as planned.
Every time you embrace uncomfortable situations in training, you will better prepare yourself mentally and physically for race day.
In order to make improvements in your fitness AND to execute on race day, you must be able to reflect properly on your training sessions AND races and to learn throughout your individual training journey. Sure, you can tell yourself over and over that you need to train harder, train more, eat better and stay more mentally tough on race day but it isn't until you begin to accept the mistakes as great opportunities for personal growth that you will really see an improvement in your fitness and performance.