My decent sleeper graduated to a horrible sleeper with no end in sight

I don't think I need much advice only reassurance that we are not the only ones.

Son was a decent sleeper his first year of life. Slept in a crib in his own room from 7 months of age. He never slept fully through but wakes gradually lessened until it was rarely more than 1 or 2, each time settling back quickly with nursing. I thought we were lucky.

He is now almost 15 months and for the past 2ish months things have gone downhill and there's no end in sight. Every single aspect got worse: more frequent wakeups (4-5 on average), needs to be nursed for long periods and/or rocked while standing before he falls back asleep, refuses the crib after the first wake (and sometimes even to leave our arms), could stay awake for 1-2 hours in the middle of the night crying and not wanting to play but also fights any attempt to be put back to sleep. We cosleep and nurse frequently at night but it doesn't make things any better, especially since he sits up and wails as soon as he wakes up each time, so I can't just offer a boob while lying down. Even as we pick him up he screams and arches his back to get away, but if we keep him in our bed he just sits and has a meltdown and nothing calms him.

The only thing that changed just before this all started was starting daycare at 13 months (I went back to work then) so it could be an extreme case of separation anxiety. In case I'm wrong we also tried all the usual stuff (teething medicine, gas drops, schedule adjustments when not at daycare, different room temperatures, doctor visit...). During the day he's a happy baby, eats well, doesn't cry a whole lot. At this point I am resigned this is our life for the next couple of years and he's going to be unhappy all night no matter what we do. We wanted to start trying for a second in a few months but of course that's off the cards now since we can barely keep it together.