I don't agree with this post. Look I'm no expert on APA or even handicap tournament play. We have one of those every week at my poolhall and I hardly ever play in them. When I do I tend to do well, but I don't really care that much, but I still think what I'm about to say applies.
The point is: No other sport or activity does things the way many pool players do things. Let's say you want to be, I don't know a good American Football player (the one thing I know even less about, lol): Nobody is going to tell you to only play games until you get better. You are going to work on your skills in segments. Tackling, throwing the ball, sprinting or whatever it is they do, lol. The reason "league only" players don't get better is that they never work on their weaknesses, because they don't have to. And they only play full games, where those aspects come up too rarely to actually learn from them. If you work on, say your draw shots for a week, you are going to progress more in that short time than you will a whole year of playing. What you learn will be better ingrained, so it will hold up under pressure as well. Sure, everybody chokes now and then, but practising in your basement or anywhere else won't make that happen more. Weak skills and lack of training will.
Developing mental toughness can only be done from within. If you practise without purpose, without pressure, you are doing it wrong! I can shoot stop shots and get so nervous I'm shaking all over, if I wanted to. It's all about the mind, setting a high standard for yourself and refusing to let up on it. I rarely play tournaments, focusing on those I think are the most important, but I do well in the ones I play in, because training by myself gives me confidence, discipline and good fundamentals. I'm still developing as a player, even after 15 years of play I get a little bit better every month.
On the other hand, someone playing ONLY tournaments, never practising, risk getting gun shy, not shooting certain shots because of a previous bad outcome, or even stifling his/her game by shooting overly conservative shots rather than going for the runout. Then getting beaten by for instance a lucky kick, reacting by becoming even more conservative and on and on it goes. Keep 'em in the chair, and their luck does not enter into it!