Here are some ideas that might help but everyone is different. To help build mental muscle, it is similar to weight training; for example, the body and the brain will build up a plateau or tolerance from what is being taught or trained. To help combat this unlike weight training there are only so many effective, creative ways to shock the muscle but poker offers many different ways to help build mental muscle.
To help with training narrate, illustrate, a journal about your session, track important information related to where, when, how, and why this affected your session, grade your sessions, the trick to building mental muscle is honesty, without it all of your thoughts well have no conviction.
What this does is it allows the brain to be shocked just like the muscle and after all the brain is a muscle for building. Yourself, and the brain are now put in a positive environment that is conducive for mental improvement, as a comparison to weight training because of your thoughts being engaged with the learning about yourself as a new focus about the game.
The new focus, on your new found honesty will lead to the right emotions for the brain to focus and learn new, revist old information alike to be used better, properly than your previous sessions. This allows a person to fix their own leaks, understand tilt better, as a couple of examples. An inchworm affect will happen from fixing the leaks in the backend of the game then forward thinking to your “A” game.
For this to help the person has to have passion, otherwise, true self-reflection about your sessions to help learn will not be achieved, it is similar to when a person first found poker the passion to learn, get better, and even build the brain mentally can be achieved; this can also help with enduring longer sessions productively. However, it is up to each person to figure out what they are looking for when it comes to any area of improvement, what works better for one person may be completely wrong for another. Glad to have met you and good fortune on your journey.