this was my daughter. odds are she IS holding it. too many experiences like this teach them that going to the potty hurts and they just wont do it. did you quit doing the miralax? i did this with my daughter as well for a LONG time. i also had a friend who's little boy wouldnt go for days on end and she used it for YEARS as well.
the issue now is getting her to get that HARD piece out - and that piece is why she is scared to go. she can feel it, it is hard and big and she is afraid it will hurt. chances are she is RIGHT - but once she gets rid of the "plug" she will be good to go. it sounds like you have done the right thing - she needs a laxative of some sort to overcome her ability to hold it.... i am SO sorry. my dd did the same thing.... day six came and we were both desperate. her crying me crying - me yelling push honey push. it was horrible. i have totally been there
are you potty training by any chance? i would put that on hold for awhile til you can get this resolved.
imo - best thing you can do is make every poop from here after super soft for her so she gets some "good"experiences under her belt . soon she will be old enough to talk to about the dangers of holding it, and that will help, but for now she will need help going.
bless her heart. i feel for both of you