To try to get her to poop earlier during the day try increasing her fiber in the morning. Fiber One bars are excellent and taste good, they work for my little guy with as little as half a bar, and oatmeal, especially with chopped apples and raisins in it, along with whole wheat toast are good as well. Make sure to give her water to drink with it as well as offering her water all day long. Water works with the Miralax, which draws water to her stool to soften it so she can go, which it sounds like she is, but because of the fear she's holding it until it happens involuntarily. Don't give her bananas or cheese which can bind, and make sure to offer her fruits like oranges, watermelon, prunes and plums, and veggies (except corn) at meals and as snacks rather than cookies, crackers or chips (not saying you do ; )
Have you tried sitting on the toilet while you have her sit on the potty? Tell her that you want some company, don't tell her you want her to try, and talk to her about her day or something to take her mind off performing. Let her see you go, so she knows it's normal and natural.
Also, I just read this idea a few days ago, what about a small new toy you can place where she can see it from her potty, that she can earn after 5 or so poops int her potty?
And try a warm bath. My nephew would get constipated sometimes when he was younger, and one day my BIL put him in a long, warm bath and he pooped, unbeknownst, in the tub. They realized it relaxed him so after that whenever he would have a problem they'd put him in a warm bath and take him out and sit him on the potty when they saw him getting ready to go.
Hopefully you can get her system regular in the day so she'll go on the potty and see it's not so bad and continue.