Why Does Logan Fall Apart

I'm afraid the answer to this question is stupidly simple and I'm just missing it.

What the heck happened to Logan after William tied him to horse and sent him out into the desert? At the beginning of the timeline, he is a cruel hedonist who is screwing and drinking his way through the Westworld park. By the end of William's first visit, He's been changed to the point that he ties Logan, naked, to a horse and sends him off into the desert.

We know that when he's found he's dehydrated and sunburnt but he also seems to be very traumatized by something. If I'm reading events correctly, it's after they return from that trip that Logan becomes a junkie and eventually overdoses.

What happened to Logan in the desert? The Logan we first meet would have kicked fought William, how did he end up so traumatized?

If you can shed any light on this for me, thanks in advance.