I'm sorry your wife has been disgnosed with breast cancer. My husband's family, both young and old, have gone through several cases of cancer in the last few years. Cancer is such an emotional disease, which fills us with so much fear and insecurity. All of a sudden life becomes so precious.
It sounds like you are supporting your wife perfectly: you're being available, patient, understanding and you love her very much.
What I've learnt is that cancer affects everyone differently. Some are able to shout "f**k cancer" and remain as positive as possible, others crumble and think the worse. However your wife is feeling she'll need support, understanding and patience, all of which you are doing anyway.
I will say one thing you should do though - don't forget to look after yourself. A lot will be asked of you in the coming months, try to get enough sleep and eat healthy.
I know this post might not be very helpful, but I hope it can give you a little reassurance that what you are doing to support her is enough.