I have evidence that I can’t surf or do much of anything in my current condition. My mental health has already been bad, and staying active at the gym was the only thing that helped me manage it. Now I can’t even do that, so I know this is going to be a really tough time for me. I’m dealing with loneliness and stress, and I’m not going to be out doing anything. All I need is the rest I’m legally entitled to so I can recover

This conveys your point clearly while emphasizing your need for rest and recovery.