They could've definitely done a better job at making the viewers underastand Luke's perspective, but as lame as it might sound, the mistery of the question is an answer within itself. It's kinda like Snoke: you don't know anything about him, except he was the reigning Sith Emperor. Do you need an entire movie showing you how he got his title? I don't think so. Same with Luke. The movie tells you that he had high hopes, but his own weakness led to terrible events, basically killing his own sister's son, and he could never forgive himself.
The tl;dr of Luke in TLJ is this:
https://en.wikipedia.org/wiki/Depression_(mood)
You can't really say "
oh, but the Luke I knew would have started again!!" - no, the Luke you knew never experienced anything similar to the events which went down with his pupils. I think he himself believed that he is some sort of flawless hero, so when he failed, the fall was even bigger; everything he believed in was shattered.