Mentality is a tough nut to crack. If you work to make a player more determined, he will inevitably become more aggressive as well as more influential...that seems realistic to me. Luckily when a player reaches skill level 15, their progress rate in that skill slows.
Too bad that
when a skill stat reaches 15, instead of reduced stat improvement rate,
it would be nice if the probability that skill will be improved in a "mixed" training program (instead of train specific skill) was reduced instead.