How do you grieve the loss of someone who sits across the table from you and sleeps beside you every night?

Bless you! Greiving is painful. It takes time to work through a loss. You need to seek help from others, accept help from others, try to remove yourself from painful places and situations as much as possible, explore new avenues and outlets, talk, talk and then talk about it. Seek companionship to help with lonliness, seek spiritual insight, seek counselling and be introspective about the situation & loss.
It's a good question. It almost sounds is if the person is still alive but not able to be a part of sitting at the table and talking or be aware of sleeping in your bed at night. That sounds like someone totally disconnected emotionally, or is cognitively not there because of alzheimer's dementia. If this person is actually gone then help grieving is needed as soon as possible from a therapist or psychiatrist.