Hi Anna, I've been meaning to get onto posting some thoughts on here for a while but knew it would be long... so sorry it's been a while.
Have you been able to rule out reflux? and silent reflux? (Silent reflux means the baby is not necessarily pukey but still has a sore tummy like with reflux).
You say she flings her body around but doesn't seem to be crying with pain. She's not settling when you just hold her though. So what does your gut instinct tell you she's wanting? Is she hungry? Do you feed her during the night? From my experience a 6mo will want 0-2 feeds during the night especially if there is a growth spurt going on. Is she on solids at all? What are her daytime sleeps like?
I'm not a huge CIO (cry it out) fan, but when all seems to be well and a baby is still not settling, I view it as my only option. Have you tried going in to her, speaking soothingly, offering her bottle/breast (sorry I don't know what she's on), and when she seems to be full (whether she has any or not), lay her down and say goodnight. She will probably cry - she's used to you holding her until she falls asleep - but if you listen carefully, it will first elevate and then by 10 mins it should be pausing or starting to lessen. If she hasn't fallen asleep (or nearly) in about 20 minutes then something else is wrong. But if the crying is coming and going with longer pauses in between, she's ok.
Briona had silent reflux until 3mos but we spent the next 3mos teaching her to fall asleep

First I went to Plunket and watched their sleep video. That helped me to see her tired signs during the day because it's generally easier to get into good sleep habits during the day, I don't know why but the evening is hardest

Then when she was tired, because I really hated CIO, I held her and rocked her and sang to her until she was almost asleep, then lay her in her cot, patting/stroking her and keeping on singing, softer and softer until she did fall asleep. It took ages. I think I did it for almost a month and the "sessions" were about 1/2 hour at a time

But it was enough to convince me that she *could* go to sleep by herself and she was okay, and I was able to go onto CIO after then.
Hope something in all that helps you. No wonder you are feeling like a wreck! Let us know how you get on.