Let me put it this way -
1. Shane Warne says "you're a c*nt and I'm gonna bowl you out"; and
2. One of Australia's current crop says the same
The first is scary because there's a good chance Warne will. The second less so for reasons I don't need to delve into right now.
Bell's weakness isn't mental, it's improvisation (skill you might say) against out of the ordinary bowlers. Against mediocre trundlers he can go hours without giving the remotest sniff of his wicket. And he can do this even when they saying naughty things about his Momma.
Bell's 'problem" (i.e. the reason he isn't regarded in the same league as Jacques Kallis), is that he lacks the ability to manipulate the very great bowlers. Although, against the merely good, he is unsurpassed in the world today. In a way he is the opposite to KP. KP makes elementary mistakes, but can dominate outright great attacks. Bell makes fewer technical errors than anyone I can see today, but doesn't dominate, or rather dominates through stealth/accumulation.
To put it in simple terms - Australia's bowlers have to find a gear beyond their usual range