There is a mounting body of evidence indicating that breastfeeding reduces breast cancer risk. In an analysis that combined the results of 47 studies, mothers who breastfed for a total of one year (when duration of breastfeeding for all children were combined) were found to be 6% less likely to develop breast cancer than mothers who had not breastfed; women who breastfed for a total of two years were 11% less likely to develop the disease. Breastfeeding for longer cumulative periods yielded even larger benefits.
The decision whether to breastfeed is very personal and should not be motivated by the desire to reduce breast cancer risk. Breastfeeding is just one of many factors in risk reduction. The stress of nursing when it is not what its best for you and your baby could be more damaging in the long run.