nearly five and still holding bowels in

My daughter is nearly five, we have been battling her holding her bowels since she was 1 1/2.  She hasn't gone in nearly three weeks and I'm at breaking point with this.  I have admitted her to the Childrens Hospital, had numerous x rays, she is on miralax and milk of magnesia daily (3yrs now) and NOTHING helps in the long run!  She drinks tons of water, eats plenty of fruits and vegi's, is pretty active (except when she's having an issue).  The peditrician just says she'll grow out of it.  Can anyone help me???

[Shylo1956]

Oh my, we are having the exact problem with our 4 + year old.  Just doesn't want to go.  She can hold it so well even with the laxatives and mineral oil.  Crazy.  If you get an answer let me know and I will for you too.  But the doctor says she will out grow it?  Avery is active and drinks plenty of fluids plus loves her apples and grapes.  It is that she just does not want to go.  I have heard of others that the child gets constipated but yours and mine is not that, right?  Just won't go.