I regard my mental strength now as one of my key attributes. Without it I think I'd score far less runs. Why, well because now when I go into bat unless you are 80+ I'm just not bothered by you, you can bounce, yorke, gob off as much as you want but I firmly believe that if I take my time and get in I'll beat you. I only built this determination on the bowling machine and spending hours with my dad bowling at me before I had regular use of a machine. Just ball after ball after ball.. Working on technique of course and all that good stuff but mainly just learning that actually, people swinging the ball at high speeds etc isn't something to be frightened of, once you learn in your mind and body how to deal with it.
Train hard, fight easy. This gives you that inner strength. So for me to maintain that has nothing to do with scoring runs (although that helps!), but more just time per week spent training/batting. If I happen to say not train for a week and rock up on match day then I'm generally a walking wicket, not because the bowling is too good but because I lose that belief that I can beat 'you', so I end up making a mistake and I'm gone.
It's another one of them skills good coaches will have, that ability to pick up what each players mental triggers are and use them. Find this guys and it'll make a huge difference IMHO. Just my opinion of course, backed up by nothing else than playing footy to a reasonable level and now learning to play cricket.