Help Mums with constipated toddlers?

My 3 year old has been going through on and off again constipation for about two years now. She lives on stool softeners and often needs suppositories. We have been to doctor after doctor after doctor all with the same result "nothing wrong with her, its just a behavioural thing...she will grow out of it". Then she seems to and she gets good with her pooing for a couple of weeks but always relapses. We have tried EVERYTHING!!!! Going back to nappies, giving her the options of using toliet, potty or nappy or even just going in her pants (as long as she goes!!!) We have tried being patient and gentle about it. We have tried ignoring it so as to not make such a big deal. I admit we have even tried being tough over it "If your tummy hurts like that you need to poo! sit on that toliet until you go" (this one was one doctors advice but it didnt work). We have increased her fibre giving her lots of fruits, prunes and juices that might make her poo, We make her drink more water, We cut right back on dairy. But nothing works. I dont know how to help her get over this problem she holds on until her tummy is cramping all the time and still refuses to go. I end up having to use suppositories which she hates and makes the problem worse because its adding to the fear aspect.. And every time I take her to the doctor its the same result, examination of her privates only to conclude that nothings wrong. More talk about increasing her fibre and water intake and stronger medication. But nothing works for long and its driving me nuts. Is this really just a faze? Or should we demanding a referal to a specialist. I did ask to see a specialist last time I went to the doctor but the doctor insisted it was very unlikely there was a need for more medical attention and we should just try giving her more lactulose and water. Aggghhhh