Is she using me as a backup man?

There could be a lot of different things going on here, but it sounds like she's trying to be free to play the field and keep you on the back burner in case she doesn't find someone, she wont ever have to be lonely if she has you to fall back on, about 80% of girls do that. But it could be that she doesn't want to be with you, but doesn't want you to go all crazy stalker on her so she's telling you there's a possibility of a future to keep you from freaking out that she broke up with you, I've had to do that one before. I doubt that it's a age issue 16 and 19 aren't really that far apart, maybe if you were 21 and she was 18 it would be a problem cuz you could go to the bars and she couldn't. In any case girls don't break up with guys they want to be with or see a future with, so you should just walk away.