Is it possible to forget your native language?

It's easier to forget how to speak it, but almost impossible to forget how to understand it. If you spend enough time out of the context of the language, it's almost expected of you to lose it. The brain is like a muscle; what you don't use, you lose (in the recesses of your mind). It'll be easier for you to relearn it though; the relearning process will awaken the parts that you once knew and add to it :)