Personally, I wouldn't assume at first she felt out of control in general, but I'd start with the obvious...
She doesn't want you to make her use the potty...and she knows full well you can't actually make her go.
So, I'd (and this worked with my dd) give it up for a while. A week or two or maybe a month. I wouldn't say one word about the potty. And then, after one morning when she dresses herself I'd say, "oh wow...did you get dressed all by yourself?!?! Really?!?!?! Wow! you sure are big!" And then, "can you show me how you can pull your pants up and down? can you do that with your pullup (panties, whatever), too?" now she KNOWS she can do it.
Then, after a few days of her knowing she can get dressed all by herself, a little kid potty appeared in the living room. I said, "hey, now that you can get your panties up and down all by yourself, you can go potty in this little potty that's just for you whenever you want, just like mama uses the big potty. see, the lid goes up and down like this."
She bit the bait and within a week was using the "big potty" with nary an accident. A few days after she was using the little potty on her own inititive, I just showed her the step stool in the bathroom in the same nonchalant way and told her she could use that to get up to the big potty if she wanted to. i didn't offer any pressure. just said it kinda off-handedly and then moved right to "so, are you ready to go to the park" or something similiar.