I believe, and speak from personal experience that, mental stress is the biggest detrimental effect to an athlete's performance.
Basically, we are a machine driven by subconscious desires, despite that we see them mostly as conscious desires. What got us here is survival. Survival came from being able to escape predation. Our tools are fright or flight. Driven by hormones, we can generate instant power to escape harm. This comes from the release of adrenalin and other hormones.
In ancient life they served a good purpose, but in modern day life, predation has turned to stress without escape. We have fright (stress) but not flight. So these stressor hormones are not rapidly consumed by the body and can do us harm.
Why, in the modern day, we can feel stress without phsysical predation is because it has become psychological predation. And much of it has come from our programming as we each grew up in our family nest. A person's character is largely generated from home, and we feel those influences in how we interact, at work and in society.
Okay, so why do the these stressor hormones affect athletic performance. Simple. If unused and remain circulating, they shut the bloodflow down in the limbs. Worse at the extremities - hands and feet (an effect called Tetany, in my experience). Reduced bloodflow means less oxygen and less glycogen delivered. Shagged already before you start. Excercise consumes and neutralises their effect, but they are still a bad effect.
So, how to rid ourselves of stress effects. Simple. A happy life. A happy life gives amazing performance and wellbeing. How to achieve it is complex. Deal with problems, not sit on them. Take a more relaxed view, chill out, laugh are some of the methods.
To assist us, we can avoid stimulants. In moderation they are okay. But alchohol, tea, caffeine etc can be detrimental. Though a happy person has much greater tolerance to soak up unsuitable foods and stimulants.
So what are the things that reduce stress, enabling us to achieve great health and performance.
1. Bodily pleasure (XES, generated Spam warning). Yes, the greatest thing that subconscious monkey left us is bodily pleasure. Why. Very clever - so we procreate and survive. Why is xes_ual pleasure so enjoyable. Simple, wonderful calming and pleasure hormones flood the body. Neat. And really healthy.
2. Good relations with people. We have to work on that. But first is don't respond to agression with more aggression. We show aggression only as a defence mechanism. If the agressor sees no threat, his agression will subside.
3. Attitude of mind. That's the chilling out part. Let it go, forget it. But mental satisfaction also comes from winning.
4. Enjoying our work. If we go to work already thinking about finishing time, then the whole day is negative. Thinking that Friday is the best part, then the whole week is negative. Negative thoughts = negative health = negative performance. Find the enjoyable parts of the work and use the less enjoyable as stages to reach the enjoyable ones.
5. Mindfulness. This how we achieve contentment in small portions. Small portions of contentment all add up. Mindfulness is a Budhist technique where we focus solely on each minute, separate actions to create a whole action. It is a diversionary technique to focus the mind. However, it develops amazing clarity of thought and prevents "mind chatter".
So, we can train all we want, but never achieve our peak performance unless we are totally happy. Work on that as much as training, using the 5 points above. And any more you can add.