Your feelings of confusion and panic (and fear) are entirely normal. Unfortunately, they make it even more difficult to get your head around what is happening and what options you have. Do you have a sensible partner, good friend or relative to accompany you on consultations? It's often easier for a calm and good companion to understand what is being discussed and to discuss this with you, probably more than once, afterwards because they are in a much calmer state to comprehend what is being discussed and to ask relevant questions for you. The more you get to know what is happening, the more you can feel a bit in control and not entirely lost. As Mez says, a breast care nurse may be a huge help at this time, ask your GP or surgeon.
It will take you some time to feel this, but a diagnosis means the start of doing something about it. Without it, the cancer just grows. So you are starting on a recovery. A mastectomy is daunting for anyone, but usually surprisingly pain free, particularly if there are no lymph nodes involved. You may be recommended to have herceptin after your surgery - either an infusion or tablets. Painfree either way. Your treatment will be focussed on reducing, as far as possible, the chance of any recurrence. You will want to think about reconstruction - some do, some don't, it's an entirely personal choice but to help you make it, your surgeon needs to advise on options, when each option is best done and of course costs and, in some states, how long you may have to wait for such surgery.
If you are working, you will need some leave for surgery (it can vary a lot, I only took a week off, I'm a quick healer) but you may want more time. It's entirely up to you who you tell - some want to keep the matter confidential (so maybe only your HR people) others are happy to let colleagues know what is happening. The same with family really - most of all, you don't want upset and anxious people fussing about you and making demands at this time. The ones who are calm and supportive are the ones you can best rely on.
There will be a time when all of this will be a faded recollection. Hard to imagine now, but many have been through this, just as confused and unsure as you, and not only survived but lived well. Keep looking forward. Best wishes.