for me it went wrong after the 2018 world cup, we had 9 players there and so had played high level both playing & mental for 18+ months
from the start of that 18/19 season many of the players were not right (trippier Eriksen were 2 that never regained their pre-WC form)
then form November onwards lots appeared to hit the wall, Poch was with the players he had there stat's he maybe should have got new backup players in on loan at start of season, maybe given players a longer rest meaning we lost a few early season games (but you need to get the supporters and the media onboard, the media because they need to fill their pages/tv/radio with content once they have the truth from the main man there is little content so they move on to the next team)