New diagnosis shocked and devastated

I don't think you're lacking courage - it's a hard conversation to have, especially at a distance, over the phone or video call or email. Mez mentioned Cancer Council Victoria's counselling service - one of the counsellors there would be able to help you with how you handle that.

For myself, most of my family is in another state, so pretty much all the 'letting family know' was via phone.  All I could do was mentally rehearse the opening lines: "I have some not-great news. Not the worst news, or anything, but not great. Turns out I have DCIS, which is..." and on from there.  I was pretty lucky in that my family is fairly low-key - when you ring them and start off with something like that, they tend to listen before launching into questions or tales of how some other distant relative had it and they're fine now.

On the mind not functioning; that's definitely a thing.  One piece of advice the nurse counsellor who was at my diagnosis gave me that has helped, especially in the first month to six weeks was "get a notebook".  Write down everything - the question you think of at four in the morning, the terms you want a definition of, the leads about treatment that a breastcare nurse might mention for you to raise with your surgeon.  When your mind is going a million miles an hour, cycling endlessly, those notes can be very grounding and helpful.

Something that's worked for me - not all the time, but a fair amount of it - was having an actual 'worry time' scheduled on my calendar. Then, when my brain started to spin out at night, I could say very firmly that I have time for that tomorrow, right now I need to sleep at least a little.  It wasn't an instant anxiety cure and it didn't always work, but it was enough to get me through, especially in the weeks just after diagnosis when I didn't know what was going to happen next.

And last, sometimes even taking it day by day is hard. When that hits, you go to taking it hour by hour, or just aim to get through this next thing.  Even if it's just 'get to the MRI appointment on time' or 'have a cup of tea and a little cry', do just that thing; whatever is after can wait its turn.