5 year old pooping herself- help!

I have my stepdaughter about 75 percent of the time and have helped raised her since before she were 2 so I consider her my first baby. Anyway, lately she's been holding in her poops and won't try to go number 2 at all. won't try to push it out, so now she is constipated. She used to have chronic constipation and was prescribed Miralax and we ran out or I would have given her some but tried 100 percent juice and high fiber snacks and everything and she still wouldn't go poop. She hasn't pooped since Saturday and since then has been wearing a pantyliner because she's had countless slightly poopy undies where she starts to poop in them but then will squeeze her butt and hold it in. I took her to the doctor because her butt is getting raw from me wiping her with baby wipes so much and I was hoping they would give her an enema or something but they ended up just prescribing more Miralax which I'm happy about and said to wait because an enema is a last resort so I said ok. anyway we go home and I said play for a few minutes while I do the dishes and she went into her room and closed the door (she never plays in her room, She usually gets a toy then brings it to the living room) So this was odd and I went in and she laying on the floor with her butt up looking weird and I was like what are you doing then she she was like I'm pooping and I was like well get up run to the toilet and poop in there! And it was too late she fully pooped herself and got it all over the toilet and pants and undies. this is the first time she's full on pooped herself since she were 4 but I'm just so over this wiping 5+ times and I don't want to her to go back to going in her undies when she knows better. like she knew she was about to poop so she just went into her room and shut the door so im upset about that and I'm tired of her trying to hold her poops in and pooping herself slightly. It's been going on for about a month and I need help. I'm due in August and don't want to be caring for a newborn and having to wipe and change a 5 year olds poopy mess multiple times a day. I've tried the whole have her help clean herself up when accidents happen but she has auditory processing issues and she doesn't understand things when you explain them to her like don't wipe up or you'll get poop in your pp, etc. So when she has helped she just got poop all in her you know where, and on the toilet, And all over her hands because she was sticking her hands in her undies. any advice????