What a shock @Ekay, you naturally have so many things to think about and work through. It's so overwhelming and s roller-coaster of emotions to work through. I hope you have good supportive family and friends around you.
Sometimes it is hard to talk to family and friends about all the emotions that go with a diagnosis so you may want to talk to a good counsellor who has experience with cancer.
You oncologist will definitely discuss treatment with you and how to manage your cancer but also check in with your breast care nurse if you have one allocated to you.
There is the private group as mentioned above and the McGrath Foundation is also an option too.
I find staying in the present moment as much as possible the best way to stay positive as you say. Not easy when we are stressed and worried but looking back to the past isn't helpful and nor is speculation about the future. Day by day is the way to go and step by step.
Have you ever tried mediation or deep breathing? I find these 2 things helpful when i am overwhelmed. Also I started a gratitude journal using an app called delightful.
If you need a bit of a break with the family then the Otis Foundation offers holiday accommdation and it is a good option between treatments for time out.
I hope others will suggest a few other things for you. I send you a big virtual hug and best wishes for your appointment.