Girlfriend Issues, "clingy-ness"?

So I am almost 20 years old and I have been dating this girl of almost 19 for a little over 6 months. I am happy with our relationship. The is great. We make eachother laugh. She listens to my paranoid rants about society and elitist conspiracies and such. It is generally a good relationship. However, at least once a week she gets to feeling very vulnerable and such and she starts using words like "forever" and other such phrases to express how she feels that our relationship should last for a very, very long time. I believe I understand the general nature of human relationships involving and I am content to enjoy the relationship in the here and now with no thoughts about the future of the endeavor, but she wants me to share in her feeling that our relationship can and should last until we die. It makes me very uncomfortable when she does this, as I have made clear to her, but the issue seems to pop up nevertheless even though she knows I feel morally uncomfortable about making promises I don't know I can keep. Is there any way to continue having this loving relationship but to avert this situation whenever it arises, or to dispel these notions of a lasting future together without ending such an otherwise wonderful relationship?