I agree with you and I also see that the weakness lies in the mentality. Almost all of Arsenal's players have great abilities, but they lack the mentality that has led to their failure to win the Premier League title in the previous three seasons.
Arteta needs to improve the players' mentality when facing competition, as they have so far lacked confidence.
Finishing second in the standings for three seasons isn't bad considering the quality of the players Arsenal have, so they need to develop a winning mentality for next season.
This is not just about mentality alone, it's just unfortunate. I don't think all of their players will have that poor winning mentality at the same time. Moreover, they have players like J5 and Havert in their midst and they have different mindset because they are UCL winners players. Again, everything is wrong with finishing second for good three seasons because they had the chance to make a difference but they took it for granted and paid for it. You can not be doing the same thing over and expect a different result. Arsenal usually start their season with a very decent and good performance but relent towards the end of the season when other teams are pushing harder thereby making their finishing at average level.