In response to mrteacher id just like to say that every case of breast cancer is completely different and unique. Some people will get through it very smoothly and although the success rates of beating breast cancer may be going up there is no guarantee that it will happen.
My mother was diagnosed with breast cancer a few months prior to my GCSE's and we were fortunately living in America at the time in the same town as a renowned cancer treatment center. With their help and support my mother got through the cancer, but as with any serious illness it was hard on the whole family as my sister and I were already at boarding school in England and were only home for the holidays. The treatments for cancer are very harsh and can make people feel very unwell.
My mother is now fine and has been clear for three years. We often reflect on it as it was a life changing time for us all, we became even closer as a family. The main thing my mother said was that the constant support was very helpful in getting her through it. Just doing things around the house that she may not be able to do are a great help for her, and talking is the best thing. Cancer patients and patients of other illnesses often get worse as they feel lonely, its as if they have been 'cut off' from the outside world and that support really really helps.
As for yourself, worrying is the natural thing to do but you still need to sleep! Keep supporting her and her family and that will keep you at peace with yourself and you should be able to sleep. The initial news is undoubtedly a shock and you will get used to it soon enough, as you being unsettled wont help everyone else.
I wish your boyfriends mother and all of your families all the best.