i do admit that it's impossible to know where your genetic limits are, but... they exist, and that's all there is to it. some guys are gonna be benching 3 plates for reps within a year or so of stepping into the gym and have 18" arms without doing much arm work. some guys are gonna be scraping for every quarter inch of size and every five pounds on the bar. sad but true.
the mental aspect will help you maximize your potential, but it ain't gonna overcome your DNA. the key is to never use genetics as an EXCUSE. just bust your ass to always improve. the odds of any of us hitting our genetic limit is pretty slim. you might not be able to get 22" arms or a nine-plate deadlift, but chances are much better that you could add a half inch or a plate per side if you worked at it.