We did formula to suppliment solids for the first 2 years, cause it was the closest thing to the advice to keep breast milk going for the first 24 months.
But then....
my daughter would hardly drink anything out of a sippy cup. And we needed to get her Miralax for constipation into her. Prune juice in a sippy wasn't cutting it either. At nursery school, they had these TEENY open cups, and she would drink some from them. They had sippy cups, the travel/disposable ones, no valves, and she would drink about 1/2 cup from those. But true sippys with a valve? she wouldn't touch. Her school thought she was drinking plenty compared to other kids, but we were worried.
Then I read for verbal late bloomers, cut them a deal. Can your daughter talk much? I was driving in the car, and was chatting with my daughter, and explained that now that she was 2, it was time for the bottles to go. So if she was to touch an animal, like an elephant or a giraffe.... in exchange would she give up the bottle?
My plan was to take her to Marine world, then out the bottles would go. She is a huge Zooboomafo fan.
"SNAKE! what about a snake?"
Yes, apparently if she could touch a snake, she would give up the bottle.
I was ready to BUY her a snake just to never wash another Dr. Browns bottle again.
So off we went to Petco. She held a snake. I googled up images of ball pythons and printed them up and put them on the fridge.
And then bedtime came. And the tears.
I knew if I gave into her once we were doomed, we had to hold the line, or it would just teach her nagging worked.
I forget how we eventually got her to sleep, but the next day was worse. She had gone from about 48 oz of fluids a day to a couple sips here and there.
At one point I was so worried about her being dehydrated, that during a nap I got out a hand puppet, threw a sheet over me, and snuck her a bottle. She drained it in less than a min and went back to sleep. She never knew I was a cream puff. The mysterious hand puppet is who gave her the bottle!
Now she is nearly 6, and she still drinks little. She hasn't ever gone back to pounding the fluids like when she was on the bottle. To take meds we mix them with chocolate milk, and have to cheer her drinking them. (or no TV until the drink is gone). She does like straws....
Good luck, it is HELL. White knuckle couple days off hell. Just bear with it and run out the clock. Talk to the school. We found our daughter would do "big girl" things at school she NEVER would at home. She was the first to lay down for nap at school, but fought them at home. Same thing with cup vs bottle. You may be able to take this in steps.