Some of the players, however, fold under pressure, especially a lot of the relievers.
Let's also look at the bullpen for what it is (and this blame goes to Theo). On paper, the Cubs came into the season expecting this bullpen: Morrow (eventually), Strop, Edwards, Cishek, Kintzler, Brach, Montgomery, Chatwood and Rosario. It looked like Strop would close until Morrow came back (we all knew Morrow wouldn't be back). With Cishek, Kintzler and Edwards the primary setup guys with Monty as the swing man and Chatwood out there to eat innings periodically and Brach to do whatever Brach does.
The only guys out of that group that haven't been disappointing are Kintzler and Chatwood (and that might only be because I had a low bar of expectation for them coming in). Cishek has been ok, but he's been hurt a couple times and just hasn't been consistent. Morrow never came back. Strop lost his fastball. Edwards lost his confidence. Brach & Rosario got DFA'd. Monty got pissy over not starting and got exiled to KC.
Early in the season, the Cubs lost too many games because Strop/Edwards/Cishek/Brach weren't effective. I'm willing to go along with Edwards and Brach folding, but Strop's problem wasn't mental fortitude, it was the loss of his fastball. Ditto Cishek, he didn't just forget how to pitch in the clutch, he just hasn't been right and wasn't effective.
As the Cubs cleared the deck, the pen has been better in the 2nd half (still not good enough). I'm excited about the emergence of Wick. I think Chatwood could be a really good bullpen arm next year if he doesn't win Hamels' rotation spot. Wieck is interesting. Kyle Ryan has been a pleasant surprise.
We also have to acknowledge that part of losing close games is failure of the offense to add insurance runs. In the Cubs last 30 games, they are 16-13 overall. In games decided by 2 runs or fewer, the Cubs are 3-7 and in those 7 losses, the Cubs are scoring just over 3 runs per game.
The Cubs don't necessarily need to overhaul the roster and they're not heading into a 5-year rebuild or anything, but they seriously need to re-tool their approach. If the bullpen, the defense and offensive consistency improved by even 3-5%, the Cubs would be cruising into the playoffs.