In retrospect it’s likely that a lot of the people diagnosed with “shell shock” would today have been diagnosed with traumatic brain injury rather than (or possibly in addition to) PTSD.
It's important to note that WW1 was very different from preceding wars; people spent weeks, months in the trenches, always on edge for incoming artillery or chemical attacks. The only thing that comes close is sieges from back when, which were its own kind of shit.