If you have learned your native language fully from childhood and speak it until you are about twelve years old, it will normally be quite stable and resist erosion. If you leave your country and begin to speak a new language, you may still experience some of the symptoms of language attrition (and this may feel quite upsetting for you and others), but it is unlikely that you will truly forget your mother tongue.
Any problems that you may encounter are more likely to be the result of a competition for attention and selection that takes place in your brain between your two (or more) languages. All of the different languages that we know are always to some extent ‘active’ in our mind. The more active a language is (that is, the louder it is screaming for you to pick it), the more likely it is to be selected by accident, even if you were really intending to use the other one. This state of ‘activation’ depends not only on how often you use a language, but also on how recently you have last used it. So, your native language may have gone a bit quiet over the years, but it is likely to still be there, and to still be healthy. Feed it more attention for a while, and it will come back loud and strong.
Language Attrition 