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.
I think we could not blame the all to just the players mentality. Yes the mentality of the players matters a lot towards the wins but it also does not seem authentic that at the same time the all players performance was not good just because of the same mentality. As someone is said over here that it was their unfortunate too along with the mentality. The game is all about the hard work the efforts and the great experience in the concerned field. The more you work hard the more chance of success has occurred. So, i think rather than blaming the mentality and luck they should focus on their weaknesses and try to overcome them in a positive way. This is the only way to fix the problems and make Themselves able to compete with other clubs with a strong performance.