Why did Team USA hurt his confidence exactly? Who thought the guy with one year experience was going to be a main rotation guy for the national team? It should have been pretty clear to everyone he was just there to provide a body for practices and to get him in the national system if he ever amounts to anything down the line. TEam USA should have been a massive confidence boost no matter what. You are among (some of) the top young players for an extended period of time and training with them.
Like if this is the level of mental fragility with Kessler that's a huge red flag.
I've never really bought the Team USA hurt his confidence talking point either. The only negative I can think from his Team USA experience is that he might not have had a chance to work on the specific aspects of his game he needed to.