The supposed Augustinian prayer to Mary is, in fact, from a medieval Bishop:
http://www.preces-latinae.org/thesaurus/BVM/OBeataVM.html
From the link:
"Written by Bishop Fulbert of Chartres (ca 951-ca 1029), it appears in his Sermo IX, De Annuntiatione Dominica. The prayer is sometimes attributed to St. Augustine, Book 10, Sermon 18, de Sanctis, since Bishop Fulbert's sermon appeared in the collected works of St. Augustine at one time. However, it is now known that the sermon is not Augustine's, but Bishop Fulbert's."
I've long known about this, which is why I occasionally ask Catholics to bring quotes from Augustine showing adoration for Mary (it's a fun "gotcha"). The quote, for whatever reason, constantly appears everywhere, but never points back to an original source, which was my original tip off.
It is necessary to question all quotations that would seem to contradict the Church Fathers on some points, as often times they are either fraudlent, or they are distorted, or the full context reveals something different.
It just never ends it seems. Beliefs based on fraudulent documentation! Still Catholics put their full faith and trust for their eternal future in a "church" that continues to use and rely on those fraudulent documents. It makes no sense.