Like you get transported into an alternate universe where everything is almost same, but with tiny differences. The world looks normal to everyone and you’re the only one that have memories of the differences.
Do you think you would trust your memories or would you think you lost your mind?
I can’t imagine you could hallucinate a person in such detail and over such a long period of time that there would never be any indication they weren’t imaginary. I would trust my memories and question the circumstances of their disappearance.
You brain makes up the details in exactly the amount you need to. That is a core problem when your brain itself is the issue.