Just joining in here as I have also been through it with DP. I'd say it's not something you can understand unless you are living with it. The change (overnight) from someone who loves you, listens to you, is reasonable, is kind to you, wants to have sex with you, cares about how you feel and wants / tries to make you happy......to living with a selfish, cold, irritable, detached zombie....is very lonely.
He has been ill for almost a year now and he has completely shut me out. I try really hard to help him and look after him but sometime he won't listen (for example to see a counsellor).
He talks to me sometimes, but it's all about him and what he is going through and I feel completely alone. I really miss my DP, I cry all the time from missing talking to him or seeing his smiling face. It's like invasion of the body snatchers.
Before this all came along, he was a really loving guy, very empathetic and supportive and I miss that. Now he doesn't know if he loves me any more or not. Hearing him say that was the worst experience of my life.
It's not the same as a broken leg or heart attack. It's a loss of the brain, the soul, the mind, the memories, the feelings. It's horrible.
I know how you feel OP exactly. It's so hurtful and just so lonely and you have no idea what to do. You keep trying but you feel like the enemy and all you want is just to see your old DP and get a hug and be told it will be okay.
My DP talks now about leaving me. He thinks the love is gone.