Just wanted to say that, from about a month down the road, my experience is that there's hope! My daughter has never been the best sleeper -- ever since that 4-month sleep regression, I could count on one hand the number of times she's slept longer than 5 hours without waking. She starts the night in her crib, then I bring her into our bed, usually at the first waking once we're already in bed. But around 8 or 8 and a half months, those early evening wakings were getting so frequent and so challenging -- we would always find her standing in her crib when we went in, and around 9pm many nights we would rock or nurse her back to sleep as usual, but then she'd be up every 5 or 10 minutes, fussing again, until we finally gave up and got ready for bed ourselves because the only way she'd stay asleep was next to me in our bed.
I definitely think the milestones, and some separation anxiety, were part of the picture. She'd started sleeping on her tummy around 7 months (after she learned to crawl) and her sleep had been getting better. Then it was like her muscle memory was programmed to immediately push up to sitting, then stand up, the moment she hit the slightest night waking.
What seems to have helped us is a real push to focus on comforting her by patting her while she's on her tummy in the crib, rather than taking her out to rock/nurse, if at all possible. It was one of those things where it seemed like she would never go for it -- she'd always ramp up from "I wish I was asleep" fussing to "get me out of this crib, help!" crying, at which point we'd always take her out and do what worked -- rocking or nursing. But then one night my husband got her to go back to sleep just from patting. He figured out that she likes being patted more toward the tail/sitbones part of her butt than the lower back -- a firm, steady, walking pace pat, we think, gives her a similar little rocking sensation to being in her baby carrier, perhaps.
Anyway, it wasn't something that changed overnight -- but probably within about a week of prioritizing patting, he succeeded that first time, and then after maybe another week or two, she now reliably goes back to sleep from patting, unless she's actually hungry. I've been working really hard on the No-Cry method of nursing until she's just sleepy, not all the way asleep, and have now started putting her into her crib very sleepy but sitting up -- she's usually nearly asleep and I help her sprawl onto her tummy and then pat her the rest of the way down.
She still wakes up sometimes before our bedtime, and we still cosleep part of the night -- but the wakings have gotten less frequent, and the best part is that she has also started napping longer consistently. I think a lot of it is just her integrating her new skills, but I think it helped that we were trying to help her learn that she is capable of lying back down and going to sleep in her crib. One thing that I haven't tried yet, but that my mom (an experienced early childhood educator) suggested is working on teaching her to "lie down" during playtime, when she's not tired and won't be crying. She is clearly starting to understand a lot of my words at this point -- knows how to turn on her "light," how to "shake" a toy, etc. -- so I think teaching her to "lie down" could be helpful.
I know this is the sort of thing that feels impossible -- the main thing I want to say is that you have my sympathy, and you're not at all alone.