I, too, combined formula and breastfeeding. We started on one bottle a day (actually, at midnight, because I couldn't cope with the lack of sleep) at 10 weeks; we gradually increased formula, and decreased breastfeeding, until I eventually stopped breastfeeding entirely at 10 1/2 months. At first, my daughter only got bottles from others, and breast from me, so she got used to the idea that dad and nana give a bottle, mom gives a breast. Then, once she was used to it, I could give her a bottle, too. You might want to try seeing if she'll take a bottle from someone else, ideally when you're not around. Once she gets used to it, you can try giving her one yourself. My DD, too, didn't have trouble going back and forth, once we got into a routine.
Incidentally, you might also want to taste the breastmilk you're giving her in a bottle. We went through a period, right after I went back to work at 4 months, when DD would refuse bottles of breastmilk, until I tasted it one day and realized that some had gone sour. I thought I was following the directions for storing the milk, but it turned out that after spending all day in a cooler, it would really only be good for one more day in the fridge. Just worth checking.
I know how difficult this can be (I had problems breastfeeding, too), both physically and emotionally for both of you. I'll keep you in my prayers. You have done a wonderful thing breastfeeding for 3 months, and you should be proud of yourself. You still clearly have in mind what is best for your daughter, and I'm sure you will find your way!