Chronic constipation?!?

My soon to be two year old (in January) daughter has been battling what I would call chronic constipation since she was about a year old... her diets very healthy, lots of veggies, fruits and proteins... she doesn’t eat a lot of carbs because it just backs her up, we’ve tried cheese because she does like it but it backs her up. Every seems to be a problem these days when it comes to bowel issues, she’s constantly in discomfort you can see it in her face, her eyes and even by the way she walks and I feel awful, I don’t know what else to do... I’ve continuously asked her pediatrician why this is happening and at first it was the blame game, as I was feeding her wrong foods/poor diet when I wasn’t so I made her diet even better and it was still happening so then it was because she’s drinking too much milk which she wasn’t and now it’s just make her drink restorolax/benefibre or anything like that which I don’t want to continuously give that to her and evidently have her become dependent on it. We dont have many pediatricians in our area so I’m stuck with what I got for the time being until someone else becomes available and I’m just so stressed all the time, everything I have to think about before feeding her, when we’re at relatives and they feed her junk I have to either be THAT mom whose “nagging” or deal with the repercussions of her eating stuff that’s going to just constipate her.. she was at my in laws for 2 night sleepover due to me having to pack the house up and was easier for her to be gone and just get it over with, anyways I sent prune purée, dates to just add that bit of extra fibre in her daily diet just because I know grandma and grandpa like to give extra treats which is no problem as long as she has something extra to help her go and then fibre and suppositories in case she gets backed up, they also know I don’t feed her cheese as it’s a HUGE binder and She’s always suffered after. Anyways, I go pick her up today and ask if she’s used the bathroom at all and they say no, so I asked did you guys give her the stuff I sent and they said no so I asked what she ate they said she loved all the cheese they fed her the whole time so now I’m sitting her, she’s beat red and balling because she can’t go and I feel just awful because why the fuck would they do that?! They know all the trouble we go through, her diets clean and high in fibre, all the pain she’s been in so like why? If they didn’t want to deal with doing any of that for her, they should’ve called me to pick her up.

I’m so frustrated and sad for her, why is this happening ?!??!