Looking for advice..

I'm here looking for some advice. My 3 year old (who is potty trained) has had extreme stool withholding for about a year. She has been to her doctor many times regarding the situation and I'm constantly told to just continuously pump Miralax in her beverages and give her rich in fiber foods .. I'm a YEAR or so into this issue and my daughter needs something more than what these doctors are suggesting.

I've tried warm bathes, massaging her back, fiber supplements, Probiotics, the Miralax, Mineral Oil, and more. Everything I can think of, I have done. She withholds her stools for days at a time, to the point she runs and hides, shakes and gasps, and cries because she's in pain but yet she will not go. It has gotten to the point she makes herself vomit, and spends a day on the couch not well from it. I've attempted a small potty chair and also the big potty in the bathroom (where she normally goes potty at). She currently is withholding bad enough she gets up in the middle of the night, multiple times, and stands with her legs crossed gasping and holding it. I am in need of advice. Thoughts, ideas, anything to help my daughter. Im worried about her insides and feel like long term use of Miralax is not best course of treatment. Thank you in advance.