L.L.
Perhaps I can help.
My youngest will be four in a few weeks. She's potty trained to pee - but not poop. Here's why:
Like your child, she has suffered chronic constipation - since she was 10 months old. We have gone back and forth with Miralax, stronger things, diet, water, everything - and JUST NOW finally have it under control. After a recent hospital stay (she has other health issues) they FINALLY convined me to leave her on Miralax for SIX MONTHS STRAIGHT and not quit for ANY REASON - and I am listening. But they finally explained it (my pediatrician never bothered to explain.)
Due to chronic constipation, fissures, etc - her colon/intestine/whatever you call it is stretched out because of constantly storing fecal matter. Now, this is a child who would go 10-12 days sometimes without a single bowel movement even though I was giving Miralax, and would leak around it, etc - don't know if your child is that bad, but anyways....
The doctors at Mayo said she has literally lost the ability to tell when she has to go because the area is too stretched out to feel the pushing of poop as it moves through. They said it would take at least SIX MONTHS to shrink back down to a point where she could really feel it BEFORE she has to go, and until then I am to leave her on Miralax because if she CONTINUES to get constipated, it will continue to stay stretched out.
This means she will be in diapers until she is four and a half, they said maybe even five - and she just can't help it. She doesn't know when she has to go. I feel so guilty for not just leaving her on the Miralax ALL THE TIME, but every time she would even out for about a month I would quit giving it to her and the problem returns full force.
Okay, so my child has CD so has a serious lack of fiber in her diet (yeah, go ahead, try to get your toddler to eat quinoa or amaranth. Sure. My toddler likes rice, I'm not supermom. That's a jab at anyone who thinks we ought to feed them better - I've tried everything.) My daughter actually loves prunes, eats plenty of them - it just doesn't help.
So we're using the Miralax, two fiber gummies a day, all she drinks is water water water, and we're just slowly waiting for her to regain that sensation.
Sorry I'm so back and forth with my answer - it's late, I am sooooo tired, but if I waited until the morning to answer I'd just forget altogether.
If you'd like more details or advice, more info that the doc gave us or what else we've been through with this and what else we're doing, please pm me and we can talk. I feel like we're finally gaining some control over this, and I'm beginning to see some light at the end of the....er...tunnel. ;)
(ETA - also, remember to praise every success. She can pee on the potty fine, and we just say things as we're changing poopy diapers like "someday soon you'll be able to tell when you have to go, and won't that be great? You'll be all potty trained!" and I try not to let her feel bad, because they're very intelligent at this age now and they KNOW that they're doing a "baby thing," trust me, and you just don't want them to feel bad because they literally can't help it.
Oh - and one more thing - for about a month before we learned that she really COULDN'T help it, I was lecturing her - things like "You KNOW how to go potty! You're a big girl! Why are you not going potty?" And I felt SOOO bad after these doctors told me that no, she DOESN'T know how to poop, and it will take her a long time to train her body in that way. THis poor kid - it really wasn't anything she was doing wrong.)