One thing I didn’t realize was being shown in Rey’s vision was Kylo and his men killing Jedi. Those were probably all the ones Luke had trained since ROTJ, and they’ve been wiped out. Luke putting his hand on Artoo was probably after he discovers this. He doesn’t just feel the pain of failing with Ren, he’s lost an entire generation of new Jedi. That’s a terrible burden to bear.
That really hardens the impact indeed. Luke has been told the tales by Obi Wan and Yoda of great numbers of Jedi. All Luke wanted was to steer those myths/legends into a reality. But he failed both his masters and his masters won’t speak to him. He has no other choice but seek the first Jedi Temple for answers, if any.