I had similar issues with the twinkies...but not at that age. It took them a bit to get used to the sippy cup, just because holding it was an issue, more so than the transition from bottle to cup.
I used the Gerber ones, that had a "cup" and a "bottle nipple", along with a sippy like thing. Getting them used to holding was mostly the problem, so I did the bottle nipple for a bit, than when they were able to hold the cup part better then put the sippy part on it. It took a bit for them to get used to it, but bottles were not an option!!!
I understand a lot of children have issues with transitioning to different things, but just keep at it. She may get upset but you have to pick your battles...either let her have issues with her teeth later because of it, or let her have a fit until she gets the idea that YOU are in control and are not going to give in.
We as parents want the VERY BEST for our children, but we don't want to harm them in the process. I'm sure you are the same way.
I agree with the other ladies about the other source of milk products, as long as she is getting calcium then don't sweat the small stuff. Same thing applies to them not wanting to eat. Children don't understand "time", like we do...we look at the clock and it says it's time to eat breakfast, lunch & dinner...so we eat hungry or not because we have learned to do that. This bottle to cup is a learning thing. Change isn't always our best friend, but sometimes we just have to do it!
Don't beat yourself up over it, you are NOT a bad mother, just at your witts end and that is why this board is here...to get and give ideas to other moms/parents in need!
Hope that helps!
Take care,
God Bless,
J. D
Blessed Momma to Ash & AJ ~ 4 yrs old