I understood that breast cancer doesn't cause death unless it has metastasised (spread) to a vital organ, and so I learned about micro-metastases. These are the microscopic cells which have shed from the primary tumour and are able to travel through the body via the blood or lymph, and less commonly, peri-neurally (via the space surrounding the nerve).
If the internal terrain remains as it was when the body originally manifested cancer (ie inflamed and acidic, with a weakened immune system) these cells are able to seed elsewhere. Yet even if cells have successfully separated from the primary tumour, survived travelling through the blood or lymph and reached distant organs without having been detected by the immune system, they still need to go through another complex process before they can grow into tumours. To survive they must create a blood supply (angiogenesis).
The most common sites for breast cancer to metastasise are the lungs, liver, bones and brain (one theory being because these organs all originate from the same embryonic germ tissue). When tumours grow big enough to disrupt the normal function of these organs, the body begins to fail. Tumours may also impede detoxification routes, or diminish the immune system to a point where the body cannot fend off a simple virus. But the main cause of death from cancer is simply tumour burden. As metastases grow, they use more energy than regular cells, and eventually starve the body of energy and nutrients, resulting in wasting, or cachexia.
Finally, and maybe most importantly, I learned to follow my instincts, ask questions, and listen to my body.