They don't pay attention to what's going on around them and their reactions slow. If you find yourself fighting with the bike and riding with your head down, make a conscious effort to relax and keep your eyes up. As you improve as a racer you'll find you get better at staying alert and in control even when very tired. Your limits are not only physical, but technical and mental too. Learning and improvement come from pushing the boundaries of what you're comfortable and proficient at.
Bike racing is supposed to be difficult and a small amount of fear is normal.
But if you're consistently stressed about crashing, or spending time constantly riding your brakes due to fear, it might be worth backing off in the race and giving yourself some space, even if you get dropped. A lot of times when I'm scared I ride right at the front, or go way to the back where there is more space until things calm down. Later, you can work on your skills and confidence so that in the next race, you'll be more in control.