The Jedi are all dead, his father was the second most evil person in the galaxy, he's correct about the Jedi letting the Sith/Empire rise, his attempts at a New Jedi Order failed and everyone was killed or defected (then vanished, apparently), his nephew was heading to the dark side, his mentors are dead (and from his reaction to Yoda in TLJ it doesn't seem as if any of them are exactly regular visitors) so, in my opinion, despair is quite fitting. That despair manifested as his split second thought about killing Ben and look where that went. Add that to all the other hurts and I'm not surprised Luke went full PTSD. Everyone freaks out like Luke was perfect. He's not. He's, at best, a partially trained, completely unsupported Jedi. Grand Master Yoda had 800 years of experience and the benefits of the Jedi at their height and when it all came down he ran off to Dagobah and went a bit strange. Yet, Luke should have remained the happy go lucky soul from the OT? Yeah, that's the realistic angle that TLJ should have taken.
And before anyone mentions Hammill's initial complaints about RJ's direction for Luke (which I've seen lots since LTJ was released but never with his part where he came round to the new direction - that's always cut off), I must say that Hammill was starting to remind me of Jason Nesmith (Commander Taggart) from Galaxy Quest. Living on past glories.
All that said, the one thing that I think TLJ did really wrong was the use of an FTL jump as a weapon. As many others have pointed out, once you weaponise hyperspace then you can never again have a fleet battle as a single ship can wipe out any fleet.
Oh, and the stupid Cantobite sequence. So preachy and pointless. I'd rather they just found the hacker in the brig and introduced the kid completely fresh.