Let’s break down Luke vs. Vader in RotJ:

Luke ignites his saber and tries to kill the Emperor after he toyed with Luke’s emotions, which clearly are one of his great weakness and everyone knows it. “Bury your feelings deep down Luke…”
Vader then defends the Emperor and Luke consumed by rage and hatred viciously attacks him, until he realizes it’s a mistake then he stops.
Luke refuses to fight Vader, defending and evading only
Vader teases Luke and triggers Luke by mentioning Leia
Luke again is consumed by rage but ultimately overcomes the dark side and doesn’t kill his dad.

What did Luke learn?

That the dark side is overcomeable, because both him and his father did so.
He has to gain more control over his emotions.

What did we learn in TLJ?

Luke’s nephew, whom he loves and has known his entire life, is being tempted by the dark side as a teenager.
Luke then goes to him in the middle of the night and his instincts tell him to kill Ben.

wait what?? what about all of Luke’s failures and how he grew in the OT and all he learned about overcoming darkness? Why isn’t he passing that forward?

He didn’t act like his RotJ self would, but most importantly, like any regular human being that progresses naturally under given circumstances would.

Luke in RotJ only attacked to protect something or someone, but he did it the wrong way, using the dark side.

And how did the Luke that knew that defying all expectations and odds can pay off simply gave up on the galaxy and the ways of the force?

I don’t buy it 😕