Hi There. I Wonder If You Can Help? My 3

Hi there. I wonder if you can help? My 3 year old daughter is going through stool witholding... This has been going on for the last 4-6 months. I have been back n forth the doctors numerous times and she is now on miralax. It's not made much of a difference and she is still holding her poo in for dear life! I am getting so stressed over it and I can see that my daughter is unhappy too.... but she just cannot seem to stop doing it whatever I try. You name it I've tried it!! She's been out of nappies since she was 20 months old so it's not as if it's because she is just getting used to using potty/toilet. She was fine up until about 4-6 months ago. I don't know where else to go with this and I have no idea how to get her past it anymore :( please help!!??