Absolutely, @Siewli - definitely don't overthink things and DEFINITELY don't use Dr Google. A lot of the stuff that pops up there is very old and will most likely not relate to your diagnosis. It will only scare you. There is lots of good info on your other post, too - so maybe read that one again, too ;)
Try & keep as busy as you can in between the 'waits' - the waiting time between appointments & surgery etc absolutely mucks with your brain even more than anything else! .... Keep doing things you love, that will help distract your mind. If you can do physical activity as @Cath62 has said - even just walking - it should also help with your insomnia.
As it is such a sneaky beast, most of us would have had their cancer growing for some time before it was 'discovered' .... Did you find a lump yourself? Or was it a GP or was it Breast Screening? What state are you in - pop it into your Profile - we may have other members living nearby that you could even meet up with, covid willing?
Just take one day at a time, one hour, if necessary.
I hope you have supportive family & friends around you to help you thru this anxious stage and your ongoing treatment - once you've got your surgery date & treatment plan afterwards - you'll be surprised at how much 'better' you'll feel.
Take care