This is maybe the wrong way to answer but I find that the better and more knowledgeable about pool I get... the more mentally strong I get too, because I know my shooting ability can overcome even my bad nerves/temper. I've seen many players claim they choke up against better players, but if they were to analyze their game ball-by-ball, or per-inning average, or whatever... the reality is they lose to the better players because they are better players. Their own playing was about the same all along.
You cannot beat a better player on 'mental toughness' alone. In fact, forget mental 'strength' or 'toughness' as a concept. Those are terms that I think best describe how you fade the bad rolls and losses. Determination and competitive drive is a different thing that cannot be given to you from a book. You either want to play seriously and win or you just don't care enough.
Your solution therefore is to make it so that those opponents are NOT better players... by becoming better yourself from a strictly technical point of view. Practice for better mechanics, better shotmaking, better planning (probably in that order). When you have the skills, the mindset (which can be boiled down to "I'm gonna f%ck this guy up") quickly catches up.
I'd throw out the mental books and get "The 99 Critical Shots in Pool" by Ray Martin, to start.