First, your GP is right -- you need a psychiatrist. Sorry, but your GP knows he's not an expert in these meds. Most psychiatrists aren't either but when you find one that works for you you no longer have to see them very often -- you only need to see them if you have issues or need to change medication. As for sleep, the problem with taking medication regularly for sleep as opposed the way AnxiousNoMore uses it as described is it makes the insomnia worse in the long run. This is called rebound insomnia. You want a more permanent solution if you can find one. Also know that withdrawal from drugs can cause insomnia, and it sounds like you've had your share of it. Also, snris such as Effexor can also cause liver disease, or how does your doc know it wasn't the Effexor? I think quitting a drug in order to take other drugs for sleep is missing the point -- if you're not sleeping because of your anxiety or depression, the meds you're on aren't working. If there's some other reason for not sleeping, such as apnea, or medication induced insomnia, or a melatonin problem, that has to be addressed separately. Effexor, as well, is a stimulating antidepressant, and that might be exacerbating your sleep problem. If you are going to try Zoloft instead of Effexor, make sure you taper off the Effexor very slowly, as slowly as needed. A GP probably won't know how to do this. It can be very very hard to stop this drug. I guess the bottom line is, if what you're taking isn't working, you either need to augment it with something that makes it work or try something else. At some point you want to at least try therapy and see if you can't beat this thing, but in the meantime your drugs should at least work. I really doubt you have serotonin syndrome, you'd be feeling a whole lot worse than what you describe. What might have happened is that when you stopped the mirtazapine, a lot easier to stop than Effexor, the stimulating aspects of the Effexor targeting norepinephrine receptors made it more stimulating to you, but I have no idea if that's true -- a lot of depressed people don't get this problem, it's more of a problem for people who also have anxiety.
You have 2 issues, depression and insomnia. I use Zopiclone on and off too for sleep but it seems to be hit or miss whether it will work. I imagine part of the problem is not meditating or reading instantly when I wake up so I can get back to sleep again, and instead thinking about things which just seems to get me fired up when I want to be sleepy.
Sleep for people who have issues requires careful management of your moods as I explained above as well as good conditions - plus you might need the meds if nothing else works.
Sorry that is all I can offer you, but i imagine Paxiled will give good advice on the meds later.