All backed up

I am so tired of watching AJ strugle with her constipation issues. I have looked for reasons that could cause it, and I can't find anything other than what we've already tried. Her pediatrician just says that as long as the Miralax works, there's no issue. Well, Mr. Pedi isn't here when she's screaming in pain trying to do normal body functions. He doesn't see the pain in her face. He isn't around when it gets so bad she literally needs help pushing it out. I've done the poop zit pop more times than I ever wanted to do in my life. I've seen her tear anus just trying to get it out. I am at a loss as to how to help her further. She's on Miralax daily, she gets a high fiber diet, she eats lots of fruit and veggies. Some days are fine, she'll go without any trouble. Then, for no reason I can see, she'll just back up and by three days without poop, she'll be strugling and screaming all over again. I can't figure out what else to do for her. I keep saying I'm going to shop for a new pediatrician, but I'm not sure that would help. I just wish there was a place I could get answers to help my poor girl.