I live with someone I don't know (and don't particularly like)

I have a 24 yr old son I can't remember.
Neither understand that almost all the memories are gone.
In family discussions remembering funny stories from the past, I have no idea what they are talking about.
When I talk to my 24 yr old, I'm uncomfortable. I don't know what to say. I can't remember his life at all. That's what hurts the most.
My son's grandmother (on father's side of family) died and I had no emotional response. I don't even remember her much.
All I have left are photos in boxes, albums and on CDs. I look through the pictures every so often. Sometimes I realize I knew those people but events and locations are completely gone.
It's been almost 4 years now since I lost my memory of what happened in my life for the previous 25+ years. Except for a few little snippets nothing remains.
My therapist says I just have to move on. I wish it were that easy.