I married my high school sweetheart, so I speak from personal experience. I met my husband when I was in 7th grade and he was in 8th grade. We started dating two years later. I always knew I would marry him. I can't explain how I knew, but I did.
That being said, it was hard to stay together through high school and college and graduate school. Even though I knew I wanted to marry him, there were days I was extremely grateful that we weren't married yet. I needed the freedom to go somewhere else at the end of the day or after a fight. Even living together before we got married didn't feel quite as final as actually being married. I still knew that, if I ever wanted it (and I never did), I had an out. I could move.
When we fought, the knowledge that I would someday marry him helped give me the strength to fight for the relationship. Some days you don't want to keep a relationship going. Some days the amount of work it takes to forgive and forget doesn't seem worth it. And there are times that one of you will believe in your relationship more than the other. Let your belief that your girlfriend will someday be your wife help you put in the effort it takes to move past a fight and keep your relationship going.
If you or your girlfriend finds that you start to get impatient to get married, keep in mind that marriage is about both of you and your relationship. The wants of one can't steamroll the wants of the other. So if one of you isn't ready to get married, you're not ready. Period. If your relationship isn't ready, you're not ready. Period. You may both feel like you want to get married and still be horribly unprepared for marriage. Only when you're both ready and your relationship is ready would I advocate for marriage. In the meantime, I would focus on building a relationship strong enough to last, because marriage is hard. (Also, no matter how long you're together before marriage, GO TO PREMARITAL COUNSELING. At the worst, it will help you identify things that might cause conflict later. At best, your counselor will tell you how awesome your relationship is.)
One of the most important skills in any relationship and marriage is communication. Learn how to say what you mean to each other, not what you think you should say or what the other person wants to hear. If it's hard to say, it's probably well worth saying. And anything worth saying is worth saying well, without judgment or laying blame or being hurtful.
My husband and I got married last May, a few months shy of twelve years since our first date. I'm not advocating that you and your girlfriend wait as long as we did! But I do think that waiting to get married when you know you've found the person you want to marry can be beneficial, as well as a very sweet time. You have time to prepare your relationship for marriage before you actually get married! Think how much stronger your marriage could be if you have a few years to intentionally prepare yourselves for it!
The decision ultimately lies with you and with your girlfriend. I hope you both decide that being unmarried doesn't lessen your commitment to each other. And I hope you fight hard through every struggle that comes your way to stay together. And I hope you do get married someday. And I hope you learn how to grow together first, so that you'll be prepared to continue growing together for the rest of your lives.