The best soccer players I have played with or coached have pretty much all had the following traits.
1. Treat everything as a competition
At the University of North Carolina, perhaps the best college soccer program of everness, the coaches track everything a player does in training using a points system. It makes everything matter, and when you train with that level of intensity and focus good habits form.
Noah Delgado, Jeremy Proud, and Kupono Low were the best players I played with in college. They were also BY FAR, the most competitive about everything from 1v1 games, to fitness tests, to goals scored or allowed in a shooting exercise.
2. Open minded about understanding their weaknesses and building them up
Michael Jordan was not known as a great defender when he was young in the NBA. He did not take the criticism of coaches and the media personally and instead and worked extremely hard on his defensive abilities. He ended up becoming Defensive Player of The Year in 1987/88 season.
3. Unwavering belief in themselves as a player
Using myself as an example, I had a habit of allowing my soccer performance determine how I felt about myself as an overall human being. If I didn’t have a good game, I couldn’t enjoy life or feel good about myself until I did. The saying “you’re only as good as your last game” was something I believed in and it was detrimental. Each time I played it was like I had to re-prove myself instead of calmly (versus anxiously and desperately) approach the game and expect to improve each time.
Great soccer players understand mistakes are a part of the game and they don't give too much credence to a bad game or even a stretch of poor performances. Another way of saying it is that they are not overly attached to a certain outcome and because they are detached, they are less likely to over-think. This way of being is more likely to lead to flow state or peak performance states.
4. Unapologetic about their game
When I attended the Indiana University soccer camp, the coach from IUPUI gave a passionate speech the first day to my group. He was adamant that his players don’t say “oh my God” or “my bad” for each bad touch or mistake. He called it a pet peeve and explained that verbally apologizing is the insecure need to tell people that certain mistakes are not characteristic of you as a soccer player. If you are good and you know it, there is no need to apologize.
5. Love for the game.
Soccer can be a grueling sport. To become great, it takes hours on end of warming up (most people hate the process of warming up), doing skill work (the way the average player approaches skill training will be addressed soon), pattern play, listening to coaches rant, training through pain and soreness, fitness training, and so on. Staying power is love of the game. Without it, the enthusiasm it takes to make progress and stay engaged will not be there. As discussed before, it doesn’t always have to be fun to be fun, and the highly motivated and mature athletes know that.