As positive as I am about this movie... & I do feel there are a lot of home runz with it... What you have said here is the thing that I'm f*cked with the most. It doesn't kill the whole movie or the progression of the series for me... but it does kind of crap on the hero I've grown up with.
@EvilDDS is there some canon novelization that shows Luke is a mental case prior to the Kylo incident? I'm not sure if that would change anything... but it's worth a look.
Also, Luke saw that defeating the Empire, led to the Empire re-building... & then defeating them again lead to the Empire re-building again under new management & changing their name to the First Order.... & so he might conceivably feel that the whole effort was in vain... & so what's the point of doing it again? I do fundamentally disagree with this premise, but I'm trying to find at least a thread of understanding as to why this hero gave up. Futility is a pretty strong argument for a hero to say f*ck it... especially on top of him failing Kylo & by extension Han, Leia, & the entire galaxy by unleashing this monster instead of keeping him on the island enjoying Porg Kebabs.