Making good decisions during hands is all that is needed.

So if a mental strength coach can help you to make good decisions (not based on emotions or feelings) then wouldn't they be of value?

Yup. And if they can help you face the demons and self-destructive impulses within, including addressing simple impulse control issues, they can be priceless. Alas, if we assume poker players are rational machines, and that they behave at the table accordingly, then we aren't even talking about human beings and we miss the mark by light years in our analysis.

A player sliding into tilt is going from the intention to make good decisions and put himself into advantageous positions ... to a frustrated desire to win right now. This catastrophic mindset includes "brain lock," in which he is being compelled into a behavior from a more primitive part of the brain that he isn't in control of. Reason or skill doesn't address this, but a meta-awareness of oneself and knowledge of oneself can. The battleground is in the impulse control realms, in the emotional realm, in the self-mastery realm. Players taking actions well beneath their skill level is ubiquitous in the game. They understand to the 15th decimal point that bad beats are part of the game, that their aces can get beat, but they do not understand and control their reaction to this. In a pregnant, intentional pause between where the beat happens, the impulse to react registers, and before the next action occurs ... is the interruption of and mastery of tilt ... reflective of the mastery of self.