A.M.
I'm sorry that you have to make this choice, but it is good that you are figuring out how to take care of yourself. Congrats on bf-ing for 10 months! Does your daughter take bottles? I hope so, because that will make it a lot easier. Also, do you pump or have any breast milk saved up? If it is at all possible, I would suggest that you switch to pumping as much as possible. If you can start introducing formula gently, it may not take too long for her to make the adjustment. You can try giving her bottles with 1/4 formula and 3/4 breast milk, and hopefully she will be fine with that. Do that for a few feedings (or days, if you can), and then you can try increasing the formula to 1/2 each. You can gauge how quickly you can make the transition based on her reaction, but be careful about moving too quickly, because you really don't want to put her off the bottle altogether.
If you think that you might want to resume nursing at some point, you can continue pumping and tossing the milk that you produced while taking the medication. If this medication is something your doctor advises you to take long term, then you should do so, knowing that you gave your daughter the very best start to her life, and that you're making sure that she has a healthy & happy mommy, too.
Good luck, and if you feel I can be of any more help or offer you more specifics, please let me know.