Pooping successfully requires a certain amount of physical relaxation, so the colon's automatic action can take over. Emotional tension can easily override the natural peristalsis that makes pooping happen, just as emotional tension can make us tighten our diaphragm instead of breathing naturally and unconsciously.
There's a very good chance that your daughter is feeling not only tension about the discomfort she may experience while she poops, but she may also be sensing your tension and frustration about her pottying problems. If she's too involved with your feelings, she won't have nearly as good a chance to allow and connect with her own.
Many parents who have reached this impasse (and it is common) have realized they have to change their basic relationship with the 'training' process. What I have seen work with a number of families I know is to allow a diaper for pooping until the process becomes easier and more automatic again. They have assured their children that they WILL be able to do this just as soon as they are ready, and they express every confidence that that will be a happy time for the child (never mind the grownup). They have asked their children to make whatever suggestions they can about what would make it easier or safer for them.
In your daughter's situation, she may well be experiencing difficult and even painful poops. Miralax seemed to help her, but did not help you meet the goal you set for her, which is, plain and simple, pooping in the toilet, and not her pull-up. I hope you will reconsider this – her health and comfort should be a higher priority here. Children who have trouble with pooping for physical and/or emotional issues often begin withholding, and can develop a much more serious problem called encopresis, in which a large ball of stool hardens in the lower colon, stretches it out until numbness occurs, and then fresh stool continuously forces itself past the blockage and causes soiling. This is a difficult situation that usually requires the guidance of a pediatrician and months of consistent treatment, usually with Miralax.
Many, many children are allowed to poop in diapers until they are ready to use the toilet. They all come to it in their own, unique ways. Positive, supportive (but not pushy) messages from parents are helpful. A diaper for pooping does not generally cause regression if peeing in the potty is handled as a positive behavior with happy results. Some parents have successfully started a transition to pooping in the potty by having the child sit, diapered, on the potty while pooping. After several successes, they cut a hole in the diaper so the poop goes directly into the toilet. After several more successes, the diaper can come off.
Pooping is commonly a separate stage of training. Some kids actually accomplish this first, but for the majority, it will come later. Sometimes many months later. If you can relax and accept this simple fact, make all your potty messages calm and encouraging, I'll bet your daughter will have an easier time finding her own way to success. And ultimately, potty success is defined by the child being in charge of the choices.
Wishing you well.