Confidence appears innate for some.
In this case, Kolb has had a tough time adjusting to the Cardinals' offensive system after four years in Philadelphia. I don't think that means he's incapable. But it has contributed, in my view, to his inability to play freely.
Other quarterbacks have struggled making adjustments to new teams even when the systems were similar. That was the case with Matt Hasselbeck in Seattle a decade ago. Before going to three Pro Bowls and a Super Bowl, Hasselbeck lost his job to Trent Dilfer. The change might have been permanent, too, had Dilfer not suffered a torn Achilles' tendon. We tend to forget how tenuous that situation was for Hasselbeck given the success he enjoyed later.
The same was true for Kurt Warner during his early years in Arizona. For a while, opponents knew a hard hit could rattle Warner, forcing him into erratic play. But as the years passed, Warner regained what had been missing. He eventually led the Cardinals to a Super Bowl.
Kolb might never get that chance. He will need an injury or poor play from Skelton to get back onto the field. And if it doesn't happen for him this season, he's likely gone. The money he commanded from the Cardinals sped up the timeline. He needed to produce quickly.
The offense in Arizona requires different footwork from the quarterback. Kolb has struggled to adjust. He has also had a hard time trusting the offense. He's been too quick to bail from the pocket, something Philadelphia tolerated and even encouraged from its quarterbacks while Kolb played for the Eagles. I think that has made it tougher for him to play confidently.