I have been thinking about your question for a couple of days now. I think I have been thinking about the question because it made me sort of sad. I don't know if this will give you any kind of answers.
I'm not sure there is a "process." Many of the police officers and military people that I know have "compartments" for lack of a better term. They have their detatched selves at work, they do what they have to do for the job and then they turn that "off" when they are at home. I also think that is why many groups like that have such a camaraderie with their coworkers and their coworkers families. It's to make up for what the job takes away from them. It's like getting an extra dose of compassion and understanding.
I think to regain compassion, sympathy, and a loving understanding for regular people takes time and hard work, both on the part of the person in question and on the part of others close to that person. Regular people can't compartmentalize the way police officers can. I think it takes a lot of help from others, true friends and family who show the person compassion and loving understanding. By showing that person compassion it helps that person to re-learn it. I think it's also a kind of choice a person has to make. To say that this is it, I'm not going to be this way anymore, I want help to fix this. It's like starting all over again to change. But most of all it just takes time.
Sorry, maybe I'm just rambling on and on. Maybe it doesn't make sense. I'm not exactly sure how to explain this. I will definitely keep thinking on it and see what else I can think of to say. Maybe other people will be more eloquent and can say it better than I can.