holds it until she is in pain!

ccoslp

This is my first post here so hello! Let me start by saying I have 6 yr old twins so this isn't my first rodeo but my 2 year old daughter is proving to be much more difficult to potty train (she turned 2 in October). She shows signs of being ready because she tells us when she pees, poops, and will sit down on the potty etc. She can hold it FOREVER which is really the problem. She will sit on the potty when I ask her to but she wont (or doesn't understand how to) release the pee or poop. It gets to the point that she holds it all day (until nap when I put a pullup on her then she will pee in it) then again will hold it all evening until she is literally dribbling down her leg and screaming in pain from trying to hold it in. We have tried having her naked all day, sitting her on the potty every 20 minutes, a timer, etc. and nothing seems to help because she just keeps holding until she is miserable. What should I do? Take a break from trying or is there some method I am missing?