Use it or lose it is definitely true. I was conversationally fluent in Thai after living there for years and speaking the language every day but now that I have been in Vietnam for 5 years, I have only spoken Thai a couple of times. When it came time for me to speak Thai during a short trip there a few months ago, I found that I had forgotten how to say a lot of things that used to just be normal conversation for me. Your teacher is correct.