Is she weating diapers? If not, let her weat diapers and don't put any pressure on her.
You may have to give her something internal to help, but meanwhile work on her emotional need to hold on. Usually this means she is holding in emotions and is not feeling free to let go. Guarding her feelings, not trusting, insedure. My son went through a similar period and I gave it little attention, but used what I learned from the book, Raising Our Children, Raising Ourselves, to help him free his feeelings. He became expressive and the problem vanished.
