Our mental preparation is not bad since 2017 but since 2012.
We were always mentally fragile.
The only difference is that in some eras we were so technically superior that we managed win CLs inspite of being a team of choirboys.
In other eras, when we aren't technically superior, we are just a normal team with a weak mental strength.
In the core of our footballing phylosophy is to play, pass and dominate.
There is no word or lessons about mentality, leaders, fighting spirit.
So, it is what it is.
It is more a cultural and historical thing with Barca.
A fun fact:
Do you know that in a CL final of 1986, between Barca and Steaua, played in Seville, a match ended 0:0 and we went into a penalty shootout.
Since Barca never won a CL prior to that final, our players were so scared and impressed that we missed all 4 penalty kicks, and lost 2:0.
A classical example of our mental strength in CL finals and key moments over years.
Our strength improved only due to having Messi to save our asses.
Without him, more or less, we were always a team of frightened choirboys who can play a nice flowing possession football, but who eventually crumble under pressure in key moments.
With or without Valverde.