Does the knowledge of a Language go away after a while?

Hey, i just wanted to know if you can forget a language forever if you dont use it for a while. I was born in NYC but raised in Finland and i used to speak finnish perfectly like any other kid there. I however have spend the last 7 years here in the states. Ive noticed that my finnish language is kind of disapearing, i have trouble speaking it and cant remember words and stuff! Im only 18 and i just wanted to know if i dont start speaking finnish again, will i totally forget how to speak it?