You hit the nail on the head. For adults, it is not the new language that causes the problem, it is the old language. The biggest hinderance to my german-learning capabilities is that I am too often in English speaking environments, such as here. My wife and I have always spoken English, because she does not want to lose her English skills. But I have news for her: the moment I finish up at my current workplace I will only talk to her in German until at least I find another job where I can get my practice in.