Absolutely you can grieve for someone who is still living but isn't there any more.
Like when a person is in a coma, there body is still there in the flesh but their mind is no longer. There are many situations where a person is living but their mind is gone.
I understand what you are saying about it seems harder this way, than if they were dead.
It must be a terrible thing to realize that there will be no more talks, no more laughing, no more anything with this person again.
I am so sorry you are going through this. I do hope you will be able to find a support system that you can talk about this and be able to help you.
Take care of yourself,
Linda