It's a good bet that everyone on the team was disappointed about losing the SB. If Brady was, at the time, particularly upset about any one aspect of the loss, it seems to me that it's at least as likely that he was upset with himself about losing control of the football on the 4thQ sack as he might have been over the Butler "benching".
Is Brady still suffering from a SB "hangover"? It's been 10 months of practice and prep, and 15 actual games since then, so I highly doubt it. Even so, we're both merely speculating on what might be going on inside the head of someone we don't actually know aside from seeing him on the idiot box.
As far as what's "wrong", the Pats are one freakish, game-ending play by the 'Fins and one almost successful comeback attempt against the Steelers from being 12-3 (or 11-3-1). Sure, the Pats have been successful on many such comeback attempts in "the past" - perhaps even most of them - but not ALL of them.
As far as what's "wrong" with Brady in particular ... In spite of Edelman "leading the league" in dropped passes, in spite of Gronk's play "falling off a cliff" (even though he was fine for the first six games of the season before hurting his back in week-6), in spite of 4 interceptions that literally went through the hand of his receivers, in spite of all Brady's "bad throws", in spite of playing the first three games of the season with only three WRs dressed for each game (all of them "2nd-tier"), in spite of Brady having three games in which he completed only 54% of his passes or less, here are Brady's completion percentages for the past five seasons:
2014 = 64.1%
2015 = 64.4%
2016 = 67.4%
2017 = 66.3%
2018 = 65.4%
Turn those four "thru the hands" INTs into receptions (Develin, Hogan and Gronk twice), and Brady has 66.9% completions. Turn just four Edelman drops into four more completions, and Brady is at 67.6%.
Meanwhile, the Pats' 2018 ground game is the best they've had since 2013 or 2012.
The Pats are Division Champs (again), in the playoffs (again), and one game (against the Jete in Gillette) away from locking up the #2 seed and a week off.
What's "wrong"? F**k if I know.