You are only 18 but you are starting to mature now, as we mature we do tend to take things more serious. We start thinking more about the future. A good job, education, making relationship decisions, raising a family and thousands of other subjects swirl through our minds.
It is time for you to really start making decisions that will affect you the rest of your life and I think you are starting to realize that.
It may be best to have a serious talk with her to see if you can determine what it is that you are really fighting about. Communication in a relationship is of utmost importance.
One of the hardest things to handle in a relationship is conflict. While a good and fair fight can clear the air and help you to feel closer to your lover, many fights are just hurtful and destructive. Fights that never go anywhere, that are repeated year after year, or that leave you feeling awful about yourself are not going to help your relationship. Those are the kinds of fights we need to take another look at, and find out what is going on underneath. This is true for any conflict that doesn't feel right, not just those you have with your lover.
With most fights, there are layers of what we mean, feel, intend, hope for, and believe, and what we actually say. We may only say a little of how we feel, or we may even say the opposite of how we're feeling on a much deeper level than the surface. Problems arise when we don't come from the deeper levels.
One of the most complex interactions we face in life is the relationship with our partners. There is often some initial spark which brings two people together, but for a relationship to thrive it requires communication, cooperation, and compromise.
Or you could just lighten up and take life as it comes.
Good luck and best wishes in everything you do.