So sorry, wondertwin. That just sucks. I know how hard it's been for me to do my same old part-time job (that I could practically do in my sleep before) and suddenly I can't manage to do it at all.
I can't imagine finally getting the dream job and wanting to flourish at it when this was thrust upon you.
I don't have good advice for concentration. I've heard some (that I can't seem to implement): give yourself a set time and amount of time for obsessing. If it's "not time yet" tell yourself you have to wait til xx:xx and you get your 5 minutes to obsess. I find it helpful to journal (I often keep a running doc going on my laptop, so I can add the thoughts). If I'm out walking and get on a roll, I will dictate into the Notes section on my phone. These help me. I've heard of snapping a rubberband on your wrist when your mind wanders or picturing a "STOP" sign to stop the mental movies. Hope you can find something that works for you.
I finally decided on Friday that I needed to let my boss know my situation. He lives/works many states away and I rarely interact with him. I actually don't like him at all and he's always been condescending to me. I really didn't want to do this, but the stress over looming deadlines and my inability to focus was making me crazy. I finally sent the email and got a compassionate response. I can't tell you how much relief that brought me. I told him I "trusted him to keep this confidential" and he said he absolutely would. I don't know if that might be an option for you.
(((wondertwin))) It WILL get better!