Nostalgic or Sad?

Some days I can not figure out the difference. I know that there is a difference as defined by the dictionary. In a nutshell, according to the dictionary being nostalgic is defined as "a wistful sentimental yearning for return to some past period. While feeling sad, well I think that doesn't need defining really. Some days I can't make out what I am feeling. Am I sad over what was, specifically with the military and the life I had come to think I was going to have. Or am I just feeling nostalgic, remembering times that were but that were destined to stay in the past. Whatever it is that I am feeling tonight I am missing what was. However, the difference tonight is whether or not I am actually sad over what I feel won't be in my future or just enjoying the good memories of before.