The 4-month sleep regression is real and I want to quit

I need to vent. Our daughter was doing SO GOOD - sleeping 5-6 hour stretches and we thought we finally had this figured out. Week 16 hits and suddenly she's up every 90 minutes again. I read it's because of cognitive development or whatever but honestly that doesn't help me when I'm running on 3 hours of broken sleep and have a work deadline tomorrow.

My manager is asking when I'm coming back full-time and I can't even think straight enough to write an email. How are people supposed to handle this?