@Dexter1,
Does your child need both her and you fixating on the mother? Any attitude you carry will show through in the comments you make when talking about the mother. If you are telling your child she shouldn't fixate but are in fact doing so yourself...your child will know, and will listen to the real message, not the one you just say with your lips.
It may be difficult, but after this many years, it is best just to let go of the mother, and concentrate on your childs welfare - as a single parent child.