Hello marvelousness, I'm sorry you're suffering! It sounds like it could be anxiety or even depression. I've had bouts of extreme brain fog and dissociation before too -- it sucks. You could be experiencing heightened anxiety because of the situation with your friend; I've always had a really hard time making new friends, so any time one had to leave it always sent me into a panic. Maybe you could talk to him about your worries, and tell him you'd like to stay in contact if/when he leaves?

Also, have you spoken to a doctor about your symptoms? They COULD have a physiological cause, so I'd recommend a trip to your GP.

I hope you feel better soon!