Take into account there are about 700 muscles standing between your mind’s eye and your gold medal.
Your vision, your mindset, and every single muscle are directly affected by your internal stressors. Many stressors are deep and can stem from childhood, and are well out of reach of mental manipulation or management.
Even on a good day, these stressors can have a negative impact on your mindset and muscles. When inner tension rises (or fear), you are taken out of the zone. The moment is lost and before you know it, you are left reflecting on how it could have been.