Hanna G. and Language Learning

A CASE STUDY
Hanna G. became my student at the age of two. She was born in Miami, Florida, and came to Madrid a few days after she turned two.
From the very beginning she showed interest in language study and soon associated me with English, to the point that we have never crossed a word in Spanish.
Hanna G. was eager to learn and showed a rare gift for English, to the point that she learnt how to read it very early.  She was soon able to express herself well and never used Spanish vocabulary.
Probably the secret was the constant use of English and the association of the language with a given person.
Grammar was never taught or even mentioned. Mistakes were corrected and she was asked to repeat the correction several times.
She was then raised in a bilingual environment where she associated either English or Spanish with certain people, with ease, without effort or much toil.
But when Hanna G. reached the age of eleven, something went awry and her eargerness to talk and communicate in English started fading away. Her long chats now became monosyllables and started avoiding me altogether.
Spanish had won the battle of the languages and surfaced as the victor, pushing English to the back seat, as a poor cousin.
Why did this happen? Probably the natural mental development that somehow forces the brain to take sides, to have preferences for one language over the other. Perhaps true bilingualism is nothing but an ideal, a chimera, an idle fancy.