Hi K.,
I feel for you, been there, done that! Every child is different so I will just tell you what did and did not work for us after 1 1/2 years of trying to resolve the same exact issue.
Her ped said it was mostly psychological and he was correct. We tried the diet like you are, the Milk of Mag, pedialax, Metamucil, miralax, etc.... with no success. We tried the sitting on the potty after meals which as I am sure you have found out is a nightmare and fairly counter productive.
My daughters started out of a bout of constipation, and just spiraled. I let her resume a normal diet and began giving her a "Fiber Choice" tablet daily, at night before she went to sleep. I stopped sending her to the potty after meals, and just let it be. The fiber choice tablets must taste good because she tries to get into them all the time. The bottle says to consult a doctor before giving to children under six, I did, he said give it a try.
It took about two weeks for her to become regular, and everyone's regular is different, for her it's every other day. It was no longer hard from holding it forever, she got over the fear that it was going to hurt, and she quit dreading the bathroom.
Giving it at night gave the body time to process it effectivly. I chose that brand though the most expensive because it is only ONE tablet per day instead of 2-6 per day (do the math and it is no longer the most expensive) and the one with the highest recommendations I could find on the internet. When we first started it took about three days, but she couldn't hold it any longer and it came out softer. Like I said, it took about two weeks but she became regular and stopped fearing the bathroom.
She has been off of it for the most part for a few months now and all is fine. Once in awhile she will say it was "owie" after she has gone (usually right after we have traveled or been out all day for a couple of days), I give her one tablet at bedtime and all is well again.
I wish you luck.