Honey,
First, calm down! Your daughter needs a calm mommy. That's why you really do need to take the Zoloft if it helps you. I am a huge breastfeeding supporter, but I also believe that a calm mother is going to do more good than a diet of 100% breastmilk.
Second, I'm not totally sure that taking SSRIs is not compatible with breastfeeding. My dh is a psychiatrist; when he gets home tonight I'll ask him to look up some studies and get back to you. But from what I've heard, you have nothing to fear. If Zoloft itself turns out to be a problem, you might be able to switch to a different SSRI that has better research to show no negative effects on breastfed babies.
Does your psychiatrist know you're nursing? Did you talke to him/her about it before starting up the meds? If not, place a call, and get some reassurance, and/or a different presciption. I think Prozac has a good track record for being compatible with breastfeeding. Again, I'm not sure, and will ask dh as soon as I can.
Meanwhile, breathe! Try to stay calm and reduce your anxiety. It will do you and your baby a world of good.