I would also add that two friends of mine were breastfeeding. Both used formula to supplement and both babies refused the breast at around 6 months and they had to resort to full time formula in the end. Both were devastated and one still talks about how it offends her every time she offers a bottle. I think formula could be the end of the nursing relationship and agree with pps suggestions to help you through this phase which I am sure it is. My lo goes through times when food is of little interest and breast milk is high on the agenda and then the other way round - although she is still fortunately quite a guzzler.
I really feel for you