Give yourself some decompression time. Take some time to just let yourself feel what your feeling somewhere quiet and comfortable. I usually go lock myself in the remotest bathroom in the house and then just let myself think whatever I feel like thinking, cry, rage, etc (have had to say goodbye to a few relatives; it's the worst part of getting old).

No matter how "prepared" you are for someone to pass, it will still hit you hard when it happens; also, when these things happens, everybody grieves in different ways - some people cry, some people flip into "deal with things mode" some people get all clingy with their loved ones, and some people isolate themselves, and some you'd never know to look at them that they're dealing with anything. However you react, trust me, everyone will be too busy reacting on their own to judge anything you do.

It might also help you to choose some task involved and just focus on that one thing. So, if panic starts to overwhelm you or there's just too many emotions to process, you can just think "right now, I just need to organize flowers" or whatever it is.

So sorry you're going through this; many virtual hugs if you want them.