Yeah the thing is I don't think it's mental. If anything they played over their heads/had really good depth for all of July and August. Then it caught up to them and additional injuries got to be too much. I mean, just losing Wheeler and Seranthony was tough, but in reality we've lost all three of our top Opening Day relievers on top of that, while Familia's replacement (Robertson) wore down. And we fantasized that Harper would come back from his broken hand and long absence with the same gusto he showed playing through the other injury, but it's not surprising that he hasn't. And then subtract Castellanos, who was starting to produce, and is basically playing hurt the rest of the year too.
At some point it's chicken/egg - the rotation got dinged by Wheeler's injury and Gibson/Suarez being inconsistent, that puts pressure on the bullpen. The bullpen's thin, that puts pressure on the rotation. Nola gives up four runs and the offense can't get anyone home. The offense jumps out to leads, it's easier on the starters.
For sure there are roster design flaws - and there have been every year. Not young enough, not deep enough, smaller margin for error.
It is probably fair to say that the Brewers and the Cardinals (and the Braves) also know how to win better than the Phillies - but not necessarily true of the Mets or Padres (though they both have managers who do).
It's painful because they have come so close (assuming they miss), and yet, they are going to end up with more wins than anyone other than optimistic fans (or at least, not the oddsmakers) thought this Opening Day roster had in it to begin with.