You don't need long. In less than 30 seconds you could do a flashback of Luke having a Force vision and showing he would need to train Rey.
Even if you wanted to have shame be Luke's motivation, that doesn't mean he has to be an angry, bitter old man. If you make Luke the same likable character he was in the original trilogy just with motivations to exile, I think you solve a lot of the problems of The Last Jedi. The Luke was saw was so different it wasn't even the same character.