I'm sorry to hear that you're having such a rough time! I'm a PND survivor myself and still on Zoloft
Before I tell you about the medication, please please *please* can I urge you to locate a Psychiatrist who is experienced in PND. It can make such a big difference if you're finding the medication isn't working, and to be honest they are more knowledgeable about the different treatments available than a GP.
You should start feeling better after 2 weeks of being on Zoloft. By 6 months the levels of medication are stable and you should be having the full benefits of the medication working for you. The symptoms often get worse just as you start it and you can have side effects such as headaches, vivid dreams and nausea.
I worry a little bit that you don't know this, as my Psychiatrist warned me about all of these things before I started taking them. Were you also advised to go onto the medication slowly - taking half a tablet for a few days first? You also need to be aware that there are other strange things that are unwanted side effects, like a stiff jaw, and you should be having a check-up to monitor this.
hope you are feeling better soon