Furthermore, your rhythm will be thrown off and your muscles will tighten even more. Finally, too shallow breathing will completely wipe you out endurance-wise and make you feel like you are in the worst shape of your life. Third, if your hands get cold you will lose that all important feel of the water. Swimming fast is about being able to feel what you're doing. Negative thoughts kill your confidence, distract your concentration and slow you right down.
Awareness is the key.
First, review 2-3 of your very best races. Think back to these events and try to remember what you thought before and during the race. Write this down. Now review 2-3 really bad races in the very same way. What were you thinking about before and during these events.