I don't really think its mental. It's not like there's a bunch of super complete next gens that just mentally fall apart in big moments. The closest thing would be Zverev with his service yips. FAA I guess also has shown mental weakness, since junior. The other guys all have super obvious technical weaknesses. Tsistipas - return, Shapovalov - return, point construction, Berrettini - backhand, movement, return, Medvedev - finishing power on groundstrokes, return position, inability to play on clay and to some extent grass, Rublev - everything but forehand, Sinner - movement/agility which then has a variety of downstream effects. Add to all of this a total lack of ATG caliber individual weapons.
I ask this all the time so I know its rhetorical, but other than Nick's serve, which younger player has an ATG caliber weapon? By that I mean weapons that can total unbalance a matchup like the Federer FH, Nadal FH, Delpo FH, Nole BH/return, Roddick serve, etc. Also lacking for most of the younger guys for some reason is the elite defensive movement and stamina of the big 3 (yes, including Federer).