The only thing you can control is yourself and your effort
These two topics alone, lack of focus and needless worry, can make ineffective all the best training in the world. Another area you might look at is nutrition. Perhaps you have not taken seriously all you have read and heard about the importance of proper nutrition and fueling during competition.
If you started feeling sluggish towards the end of the tournament, improper nutrition might be the culprit. Were your physical skills adequate? Maybe a physical skill you hated to practice turned out to be an important one to have had on that day. Sometimes athletes don't want to face possible mistakes because they convince themselves that, "I am different", "I don't have time to do that", or "I can't eat/drink that much". Refusing to accept a limiting factor as contributing to your overall performance is like "putting your head in the sand".
Beating yourself up after a disappointing performance is also counter-productive. The people around you, your family and friends, who have supported you through all your training and tolerated your absence at family functions don't want to hear about it. One thing you need to remember is that if you were a decent person who was loved by family and friends before the match, they will love you after it too, no matter how you finished. Losing is not who or what you are. It is just something you are experiencing at the present. You are much more than one athletic performance.
Even the best athletes in the world have experienced disappointing loses. The famous baseball sluggers have more strike outs than home runs. The great NFL quarterbacks sometimes lose and lose big. Professional and elite athletes have learned how to benefit from their mistakes and move on; frequently to better performances in the future.
The famous motivational speaker, Anthony Robbins said, "The past does not dictate the future". Many well known athletes have had a disappointing season or two and have come back stronger and better. Even the well known Peter Reid, winner of the Hawaiian Ironman, experienced two disappointing years in a row but returned to place second at Kona in 2002. In fact, on close inspection you may discover that things aren't so bad after all. Losing one big race does not make the entire season a total loss.
Sometimes it is just plain hard to find an explanation. Conditions beyond your control can determine your fate. Work related or family pressures can interfere with your concentration and training. A chronic injury or late season illness, which may have ruined your taper, are conditions that are hard to come to terms with. Realizing that you are not the first person to have experienced unfortunate circumstances might not give you much comfort but you are in good company. The real champions of the world are not always the ones who win the most awards but they are the ones that know how to rebound from adversity. The former mayor of New York City, Rudolph Giuliani said, "There are important skills to be learned from picking yourself up after something goes wrong, to keep moving ahead without letting it throw you off course".
Failing to meet your expectations can make you feel sad. Be a little sad if you must but try to gain something from the experience. You might have learned a very important lesson that you would not have learned otherwise. Make the most from the learning experience and move on to be better next time. Every time you compete you learn something useful. That is why experience is so highly regarded. Most of the best athletes in the world, in every sport, worked their way to the top, often losing frequently along the way.
"Good timber does not grow with ease: the stronger the wind,
the stronger the trees"
- J Williard Marriott.
If your performance was less than you expected, analyze what went wrong and learn from it. Soon you will be training for a new season. Accept the challenge and try again. Next time you will be armed with some new and valuable experience.