Your reactions are completely normal, you have just had to face something you were probably not at all prepared for and feeling stressed and worried is a pretty sane reaction. That said, worry doesn't help much so deep
breaths and keep your energy for beating this.
Your appointment with the surgeon will probably clarify matters and give you something more definite to focus on. Curiously most people find having more, professional, information helpful. Otherwise stay away from Dr Google or unmoderated sites. There's a lot of misinformation out there and a stressed and unhappy mind can imagine all sorts, including things you haven't got and that may never happen. Come here for advice or to let off steam, it helps! Not a group you want to be part of, for sure, but many have found it very helpful.
If you can, take someone with you to your first appointments, there's often an awful lot to take in and you are nor in the best situation to absorb it all or accurately.
There is an entire group here on reconstruction and I am sure you will find that helpful (I stayed unreconstructed!). A mastectomy is usually very straightforward and remarkably pain free.
Keep breathing, think positive, best wishes for the 8th.