Funny thing about meds... they typically mask awareness. Pain is usually the first to get masked.
You are right about psych meds -- they are pretty poisonous. It's your liver's job to break down poisons. It's probably in pain just to make up for all the pain you didn't feel over the last seventeen years. (My completely amateurish opinion.) As you fast, your body is beginning to flush out these toxins from places where they've been concealed (i.e. fat stores), and they will make you feel like crap (never mind the pain). That kinda goes with the territory when detoxing. Nothing to it but to do it.
You could probably benefit from liver flushing. From the sound of it, you're long overdue for a few flushes. (Back pain -- especially around the shoulder blade, goes hand in hand with liver and gall bladder issues, BTW.) Have you ever been checked for gallstones? You might actually be having a gall bladder attack. Try drinking some Epsom Salt
(say, 1 T in 1 cup of water or fresh [granny smith] apple juice) and see if your pain subsides. If so, repeat periodically as needed. (Don't do this for extended periods of time, however. Read the Epsom Salt
label for details.)
You might also want to do a castor oil pack -- get some cloth, soak in castor oil (or pour some castor oil into it or on it), place the cloth over your liver area, put something soakproof over it, like cellophone, and then put a heating pad over that and sit for an hour. Repeat as needed.
Also, just in general, liver is helped by fresh radish juice, wormwood, sage, dandelion, goldenseal, and milk thistle (especially). Consider getting these into your diet somehow. (Since you're juice fasting, get some fresh radish in there.)
Please let us know how you progress, or what you try and what happens.