I understand your fears, having been diagnosed myself with breast cancer when my children were just 5 years old. It is devastating. But remember, breast cancer today is not an automatic death sentence, though I'm sure it feels that way right now for your family. As they told you, many times it can be treated like a chronic illness. Use people like Lance Armstrong, Elizabeth Edwards and Tony Snow as role models for your mother to see. Many, many people today are not only surviving, but thriving with metastatic cancer, some you didn't even know had it.
http://www.acor.org/tcrc/lance.html
As you begin to talk about it, you'll hear lots of inspirational stories from others (along with the bad ones). The first story I heard which made me feel better was from my children's kindergarten teacher. She told me her sister was diagnosed at age 27 with Stage IV breast cancer and 17 years later was doing great. She went on to have 3 children after cancer even. I held onto that story and when I'd get nervous I'd think about that and other great stories I heard or read about.
It helps to get some support with coping - our medical teams are great at helping our physical bodies but not our minds. And sometimes it's the mind that's the hardest thing to heal. Some people benefit greatly from cancer support centers who offer counseling services and run support groups, etc. Others cope on their own pretty well, but be sure to allow her to discuss her fears or she may bottle them up and become depressed like many do when they're going through this type of health crisis. Having gone through it myself, I can say I am so glad I found my support center when I did. I was laying awake all night worrying about everything and the social worker and nurse at my center really helped me learn to channel my worries using a better way of thinking. There is a book called The Wellness Book which has some excellent suggestions for coping with illness if you want to take a peek at them.
I will pray your mother will be one of the long term survivors. If you or your mother need support or inspiration, we have a thread going just for breast cancer survivors and those who love them. Feel free to drop in anytime.
http://www.disboards.com/showthread.php?t=810357&referrerid=70088