Concentrating is hard work. Building that concentration/relaxation muscle is also hard work. And it's tricky, because from the outside, it looks like you're just sitting there doing nothing! It can be confusing that it sometimes feels so difficult to tame/train/educate our monkey/thinking minds. The article below talks about some of Robert Sapolsky's research findings on the mental effort of high-level chess players, who are also exercising their concentration.
From an ESPN article:
In October 2018, Polar, a U.S.-based company that tracks heart rates, monitored chess players during a tournament and found that 21-year-old Russian grandmaster Mikhail Antipov had burned 560 calories in two hours of sitting and playing chess -- or roughly what Roger Federer would burn in an hour of singles tennis.
Robert Sapolsky, who studies stress in primates at Stanford University, says a chess player can burn up to 6,000 calories a day while playing in a tournament, three times what an average person consumes in a day. Based on breathing rates (which triple during competition), blood pressure (which elevates) and muscle contractions before, during and after major tournaments, Sapolsky suggests that grandmasters' stress responses to chess are on par with what elite athletes experience. "Grandmasters sustain elevated blood pressure for hours in the range found in competitive marathon runners," Sapolsky says.
Knowing that you're working hard every day as you practice, be kind to yourself – persistent, but patient.