let me just say first, please do not give up. believe you can do this and it will be easier to deal with. i ran into the SAME exact problems with my 1st son. we gave up. i regret it now still 3 years later.
i went into BFing with my 2nd son with the mindset that i could make it through!! and 6.5 months later we are going strong with bumps in the road as well at times.
could it be thrush?
could she be gassy?
is she refusing the binky at that time as well?
are you getting engorged, maybe it's there is a slow let down?
here's my opinion, formula isn't the devil but it definitely isn't a good thing to throw into the mix. i feel like im always "anti-formula" but i have just seen in my experiences and others that once formula is introduced it's only a matter of weeks before the BFing relationship is out the window.
so stick with it!! (((big hugs)))