Is zoloft the only med you've tried? I know there's some debate about Lexapro and nursing, but I have several friends whose doctors were OK with Lex while nursing.
Prozac is also an option, one which I can personally vouch for. It has worked wonderfully for me, and I took it while nursing with my doctor's blessing. In fact, my doctor prefers prozac for nursing moms because in his experience he has seen fewer complaints of weight gain and other side effects.
Definitely talk to your doctor if you have reservations about the zoloft or if you think it's not working. You definitely have options.
I'm not sure what your long-term plans for nursing are, but weaning at 8 mos is an option as well. I'm guessing your baby has just recently started solids and could be nursing less - it could be that your hormones are out of whack, so keep that in mind too.