The situation you describe is usually not about the relationship. It’s about the timing. You haven’t been really single since you were 18. On top of that, you had a painful experience with the last girl. It could be that you didn’t give yourself enough time to recover and to assess what, if anything, you could have done differently. That’s important. When done well, self-reflection helps a person like you regain confidence and understand where you had some power and, yes, responsibility, for how things turned out.
At your age, it’s also important to gain enough experience with women that you know what you do and don’t want in a relationship. That comes from getting to know women as friends as well as as lovers. It comes from that old fashioned activity called “dating,” just going out with some people a few times to have a good time and to also learn what kind of women you connect to most easily and comfortably.
Your current girlfriend might be wonderful in many ways. But if you are still reacting to the last breakup, if you still feel like you could be taken in and hurt again, if you don’t really know what kind of person you are looking for, you may not be ready yet to be in another exclusive relationship.
In fairness to the girl, I think you should take a step back from exclusivity. Let her know how wonderful you think she is. Reassure her that stepping back is not about her. It’s about your desire to learn more about yourself and relationships before you give someone your heart. She may be willing to wait it out. But if not, it’s okay. Once you are ready, you’ll find the partner you want and need in your life.
I wish you well.