Now to begin with I want to stay that Peter Walsh doesn't actually have PTSD having not really fought in a war, but he lived in India in the years immediately following World War I and people around him were probably going through it and I think that that rubbed off on him as from what we see him thinking about and what I know about PTSD he seems to exhibit similar symptoms. When he came back and talked with Clarissa what went through him mind was just like what goes through peoples mind when they come back from war like, "how are these people living their lives just like normal after what happened to me?" Even though such a terrible thing happened to the people fighting in the war the people in the place you fought for doesn't even act like anything is different aside from congratulating you for fighting for your country.
Not only that but he seems to really regret not staying with Clarissa, along with being really disappointed she didn't love him back. But he appears to have fallen in love in India though I'm not entirely sure what he said as he sounded like he was divorcing the woman he just met, as he seemed really sad while talking about a divorce. He also chased after some unknown girl, which could've just been a figment of his imagination showing him the exact woman he wanted to marry, and showing him how he could never obtain her.