I am 39, have clinical depression and have a 16-month-old son. He is beautiful and happy and curious and perfect (I think - but I'm partial!) My husband and I are both physicians and we did a lot of research (on our own, with a geneticist, with my OB/Gyn, with my psychiatrist, and later with his pediatrician - I tend to be somewhat anxious, too!). We decided that the benefits for me of staying on antidepressants during my pregnancy outweighed the risks of exposure for our unborn son. Parenthetically, I had a relapse of my depression while pregnant even while on medication. I ended up breastfeeding him for a short time, but had to stop (unrelated to the psych meds). If I could have continued to breastfeed I would have, I think.
The medication I was on then and am still on now is Effexor, which is an SNRI (similar to, but not exactly the same group of drug as, Zoloft, which is an SSRI). There is no good data on antidepressants and pregnancy or breastfeeding, only registry data (which means women who are already on certain medications and are pregnant or breastfeeding are followed and outcomes are recorded). It's the only way to get this type of information because it is not ethical to conduct scientific studies on pregnant women. There is not a huge amount of data, either, but the most data out there is actually on SSRI's. In any case, I don't have exact numbers for you, but the dose of medication that babies get through breastmilk is something like 10% of what the mother ingests. That said, there does not seem to be a correlation between long-term adverse effects (behavioral or academic) and exposure to SSRI's. Now like I said this is not exact information, nor is it medical advice, but Zoloft is probably pretty safe as far as antidepressants go in breastfeeding.
If I were you, I would talk to your baby's pediatrician and your own doctor (psychiatrist), and even go see a geneticist to get more accurate information yourself and make and informed decision. You may decide that your are so miserable (you can barely take care of yourself, let alone your baby or even just that the depression is affecting your bonding with your child - which I personally think has bigger ramifications for your child in the long-term, than exposure to Zoloft) that you really need the medication to function properly. In that case, then you have to decide how comfortable you are exposing your child to even a small dose of medication. If you're not sure, then you should abandon breastfeeding and focus on getting yourself out of the depression. If you're OK with it, then continue with breastfeeding. Don't be surprised if your pediatrician tells you not to do it. They are operating from a certain vantage point (what's best for their patients, ie. a child, given the scientific data available - not to mention medicolegal considerations) and are not necessarily concerned with you (you are not their patient). So, your depression may not be at the top of their list of concerns.
Unfortunately, there are no guarantees and it really is a risk-benefit ratio you have to consider. If you're not incapacitated by it and feel strongly about continuing to breastfeed, think about non-medication treatments (talk therapy, UV-light treatment, exercise/yoga, etc.).
I hope I didn't overwhelm you with too much information, but I can completely relate to your situation. You just have to get the information and do what feels right. Hope this helps. Good luck and enjoy your baby!