I'm going to go another way than just making you feel better about giving formula. Of course, it is perfectly fine, but I just thought I'd mention something that I didn't notice anyone else mentioning.
I don't have any experience with this, and don't know your medical problems, but I WOULD suggest seeking other opinions just in case there are other options for you. 8 weeks is so young to wean. However, if your health is at stake, you need to be a healthy mom for her.
Again, it never hurts to seek other opinions, even seeking alternative medicine, to see if there is another option besides this medication. Also check out kellymom, to see if your medication might be ok, or if there is something else that might be and you could consult with your doctor. http://www.kellymom.com/health/meds/index.html Very often doctors use one way because it is what they're used to, but there could be alternatives. If you really don't want to wean yet, then I would definitely suggest researching what is possible.
Also, it might be possible (depending on your situation) to pump to keep up your supply, and then pick up nursing again when you can.
Good luck, and remember, if you DO have to wean, it is ok. Having a healthy mama is most important to your little girl. But I definitely understand why this is a struggle for you.
***And I have to disagree with the statement of nursemaids in history. Wet nurses were often employed in wealthy households because the matriarch needed to be able to get pregnant again to produce an heir. They were also used many times when mothers died. It is not just that babies were handed off to a specific woman to nurse. There were always reasons.