That's really interesting Pooka. Thanks for sharing that with me (and everyone else).
I had a checkup with the doctor a couple of days ago. He asked me how things were going and all that other doctorly stuff and after that I said to him how tired I was now. How I feel like I'm tired right through to my bones, to the point where if I have to hold Max while standing up for more than five minutes (I fixed this problem by buying a 'sling' for him to sit in that I can just slip over my shoulder) or so I get the shakes. So he asked me how long it'd been since I'd had a proper nights sleep prior to starting on the zoloft and I said it'd been probably 12 months or more.
He then explained to me how if you don't get enough sleep for long enough, your body builds up what is called a 'sleep debt' which basically means that the only way to feel better is to pay it back with sleep. It was a relief to know that that's all that's wrong with me. I was starting to get worried that I should have been feeling stronger physically after a couple of weeks of relatively solid sleep. Apparently it can take months to 'pay back' what you owe the gods of sleep though. I guess I now know what weary to the bone feels like.
So all in all, things are looking up. Max is a lot more relaxed now that I've stopped stressing out and his feeding is improving every day although we still have relapses when one or both of us lose the plot for a bit, but it's getting longer and longer between incidents.
Once again, thanks for all your support and advice. You've all been great.