It is very hard to give advice without knowing either of you. From a personal perspective being diagnosed with cancer has affected me in ways I would never have thought possible.
The treatments can destroy your confidence and the ongoing side effects of any hormonal treatments for many years after just add to this. Add to all this the realisation that you have looked your own mortality in the eye, it changes everything. Some of these changes can be positive, some negative, but it takes many months or sometimes years to reasses your life and future needs.
Maybe she needs this time to reasses, to decide where she wants her life to go now that she has faced this awful illness. Maybe she will come out the other side, knowing you are still there for her, and come back to you, ready to start your lives together again. But maybe her needs and wishes have changed too much for her to consider the relationship viable any more.
All you can do is give her some time to come to terms with how her life has changed, make sure she knows that you are there if she needs you and hope.