What is a disorder that can be caused by loss?

A type of psychosis but not Schizophrenia.... There is much more to Schizophrenia than just psychosis and it's NOT caused by grief. It is actually cause i late adolescences when the brain goes through a process of clearing out (destroying) excessive nerve tissue.... in a schizophrenic it destroys too much...

Psychosis can be caused by many things.... Schizophrenia, Bipolar, depression, drug use, severe stress or anxiety, lack of sleep, and there are a dozen medical conditions (including brain tumors) that can cause psychosis..... none of them fit like you want.... and all of them have other symptoms that would not allow a person to simply hallucinate another person by be perfectly normal in all other aspects.....

Delusional.

Grief. everyone experiences grief in different ways look up the 5 stages of grief and one of them is denial which you could associate with your character

it may be just a phase that people go through when you lose your closest human being. Maybe depression?

PTSD maybe?