I too, could tell by my surgeon's face that there was more to it than a simple 2.5cm tumour. It turned out to be two joined together adding up to just under 5cm and Stage 3. I had 5 out of 17 lymph nodes affected. I'm almost finished my six months of chemo and will have a mastectomy at the end of the month before 5 weeks of radiotherapy. I am optimistic that I will have many years of active life ahead of me and that's not a Pollyanna attitude to have. It does rearrange your ideas about what you think is worthwile being active for though, and that's not an easy process to get through and I haven't finished figuring it out yet. Be very, very kind to yourself. If some goal seems suddenly out of reach for now, just give it a bit of space for now, there's no hurry.
Like you I did have some fairly doom-laden feelings early on when I realised the lump I had was not a cyst in the ultrasound. But amazingly, I've had few of these during treatment, because a few things start to come together that you can't see at first and it's people, social support, information, and that one step at a time thing going on leading to you feeling that you can do everything you need to do after all and that you ARE in control of your experience.
I do feel in control for now, and that's partly because I can come here and ask for information, read other people's experiences and know what to expect. I can read the BCNA handbooks which are excellent, and download snippets from the links on the website and they help me when I want to ask questions of the nurses at chemo and especially my breast surgeon and oncologist. I've had a very supportive family, meetups with friends and walks with my dog. I'm always impressed by the people I meet when I'm in chemo, the nurses are utterly fantastic. Everyone just gets on with it. It all helps amazingly well.
Find someone who knows what you are going through (to manage the info overload!) and have a good chat. By far my best support and resource has been a McGrath Foundation Breast Care nurse, but there are also volunteer breast cancer support groups around who have great people to chat to as well. Posting here is an excellent move as there are so many wise and compassionate people. :)
Sorry for the long post, I get so enthusiastic (especially since I had chemo today and the premed that you take to reduce side effects makes me quite the speedy rabbit :)). What you need to know is that people will care about you, and amazingly, you will get out of bed and keep going through good days and bad. Walks and talks are so helpful with fatigue and crap feelings.
The very best of good wishes, Meg xxx