I think the prescription for any 13 year old in any sport is to have fun, once it becomes a business and your job, for many, the "fun" part becomes tougher, though I believe in having fun on the job, the stakes become higher.
My son loses focus when things aren't working, and as we know often times for pitchers, things don't always work everyday. Through working with a sports psychologist, instead of trying to figure out what he's doing wrong, in a matter of seconds to regain focus, breathing excercises have greatly helped in finding the right stuff he needs right away.
I agree, when we have interest, it's easy to not loose focus, when things get tough or boring one looses interest, and sometimes too much thinking takes place (for hitters and for pitchers). My son has essentially thrown out the analyzing and concentrated on over all well being. It has helped.
Now if we can just get the nagging ache in the shoulder squared away, he might get it all right.