Omg, I just added player selection, formation, ref as one of the reasons. The fact that I said motivated team when describing inter should be indicative that I also think that we took this game lightly (at least less important one than Inter players did) which was also part of the problem, eventhough I think selection of the players was somewhat a bigger one. When you field 3 mids that have never played competitive game together, when you field 20 yo defender that has barely featured so far you're looking for trouble. This kind of collapse would have never happened in the CL or Serie A, since we give a damn about those competitions and players are more motivated to perform.
I've read somewhere (was it here?!) that we're mentally fragile and that just

This is a team that come back from 0-2 against Bayern, that has piled 15 consecutive wins, that has 16 wins in last 17 games in the league, that hasn't conceded for 8 games in a row...