There is a significant difference in the brain function between people who learn a second language in their early childhood and after becoming adult. In other words, there is the critical period of learning language.
Before the certain age, there was a possibility to be a bilingual person. However, I am already adult, so my brain activity changed long time ago. I have to give up learning English perfectly.
Even if I study English for 100 years, it is impossible for me to use English like a native speaker; however, the author also said studying second language continuously makes us using it better.