Am I forgetting my native language?

I am Greek and since I reached B1 stage in English (though I doubt I was only that proficient with the language) I started thinking in English and because I would think in both languages when I spoke Greek I would switch to English or use a word and forget the greek word. The fact that both my parents used to live in English speaking countries doesn't help because my mother would speak to me in English since I was 2. I am currently literally being groomed to go to America because I have an American passport and I am afraid I will forget greek, or my level will remain to that of a child.