Arsenal had failed to maintain their consistency due to injuries and most likely fatigue too as the same players were the ones playing all the time. But I don’t really get what you meant by Arsenal not having a winning mentality. I think they’ve got a winning mentality and that’s why they had been leading the league table for a while.
One of the reasons Arsenal failed last season was due to Arteta poor management in the crucial stages; and somehow it passes everyone radar. Arteta as arsenal boss made some changes that disrupted Arsenal game play and kickstarted the poor performances. One is rushing Jesus back from injury, then benching Fit and firing Trossard for the just getting fit Gabriel Jesus; second poor decision was refusing to drop or rest Saka despite heavy and obvious burn out from the right winger. With better squad depth; i hope he improves in that aspects.
I don't think it was Arteta's fault alone because if you put all the blame on Arteta indirectly this also creates a condition where that good start to the season cannot be forgotten either.
Arteta may have had a few things that got Arsenal off their feet but when it comes to one season of form he has been very good and the end of the season was a failure all together and it is unethical to blame only the coach.
The players lost their mentality and lost games in a row and Arteta certainly didn't expect that either because he was so close to the championship initially it was just that the end of the season didn't go well.