Well, it does make sense for what your teacher had told you that 没 is used for complete action. So in the sentence 我没有去過美國, it means you haven't been to the U.S. in the past. However, if you switch it into 我不去美国, it sounds like you won't go to the U.S. for some reason. For example, after a terrorist attack on somewhere of America, you can say: 我现在不想去美国,which means you don't want/plan to go to the U.S. recently. Hopes this helps. :)