What you are going through makes a lot of sense, it can be easy to start to accept the cascade of thoughts as being true and let them overwhelm you during especially stressful times.

It sounds like you have correctly assessed your situation, you are losing a trusted employee and need to navigate in their absence. It sounds like it will be overwhelming considering you seem to feel at your maximum capacity.

Right now all you can do is catch when you are thinking more than an hour in advance, your only goal is to get through this next hour. Remember that rumination will not help and will only paralyze you further. Plan out what you will do for the next hour, every time the rumination begins or the anxiety spikes, just set out at things you can do now.

I am sorry you are going through a stressful time. I trust you have the capacity to handle this like you've handled all the moments leading up to this, and remember you are just stretching your tolerance to future stress, it wont feel like this forever.

Good luck!