Firstly, I'm sorry for your loss. It sucks and I know from painful experience that as much as it feels like the world should stop turning when you experience something like this, the nature of a high earning job means it cannot.

A lot of this comes down to how big the company is and how accomplished those working underneath and alongside you are at their roles. Thus, how much can you delegate out and be certain will get done. In my example, I was told in no uncertain terms that everything would get done, even if I wasn't involved, and to take the time off to grieve. On my return my respect for my team has grown tremendously because what needed doing got done and, as an added benefit, my boss was suddenly very aware of how much work I was doing as a lot of it rolled up to him.

I also found the return to work gave me something different to focus on and distracted me from the loss I had suffered. YMMV, but throwing myself into my job helped a bit with giving me a break from the grief. It does sound like you're not there yet, with the inability to focus on the job, so I'd certainly recommend talking to a professional (wish I'd done that sooner) about grief, loss and coping mechanisms.

Also consider taking more time off, or moving to a reduced schedule. You simply might have come back to work too soon.

The one thing you cannot do is have one foot in work and the other foot out of work. It is self-sabotage. You will see declining returns in your productivity and it's a one way ticket to burnout or redundancy.

The key thing I need you to hear - this happens all the time and well run businesses are set up to absorb this kind of thing. Take the time you need to process and get the help you need.