MANY young children do not like the potty because they feel like they are going to fall into it. Even a a potty chair, they do not always feel safe on it.
. That big seat is not comfortable and does not fit them.The children are expected to hold onto the toilet seat instead of being able to just sit on it.
Go right now to your own toilet, lift the lid up and sit on the toilet bowl and try to make a bm while holding yourself on with your hands. You can see this is what it feels like to a child. Even with an adapter, it feels unsteady.
Also many people need to be able to push with their feet when they have a bm. Make sure she has a step or something her feet can sit on and she can push against. Make sure what ever type of attachment you have on the big toilet does not move while she sits on it.
Also most people cannot be stressed and make a bm, so she will need some moment of calm, to be able to make a bm. There are many, many children that are not potty trained by the time they are 4. Take it easy on her, she will catch on.
Our daughter caught on at preschool, because the potties were all lined up and she could see the kids going on their own. It seemed to make more sense to her when she saw other people her size be successful on the potty. Let her have some books to look at while she sits there.
I am sending you strength. I can tell you are stressed, but she will get there.