There are a few different ways to answer this question, but the most common answer is that the hardest sport mentally is either boxing or chess. Both of these sports require a high level of mental focus and concentration, and both can be extremely frustrating if you make even one small mistake.
Boxing is often considered the harder of the two sports mentally, as it requires not only split-second decisions but also a great deal of physical endurance. Chess may require more strategic thinking, but it is also a much slower-paced game where one misstep can often be corrected. In boxing, one mistake can often mean getting knocked out or seriously injured.
Both sports require immense amounts of discipline and dedication to master, and both can be incredibly rewarding for those who are able to do so. If you're looking for a challenge that will test your mental fortitude, then either boxing or chess may be the sport for you.