You are very wise to ask this question. You and your boyfriend have become quite co-dependent. On the one hand, it feels great to be in love with your best friend. On the other hand, spending time exclusively together puts too much of a burden on the relationship. Further, it keeps you from groiwng out of your shyness.
My advice is this: Start by getting involved in something you truly care about where there are other girls your age. Consider a service project, a sport, or a school club where people you find interesting hang out. When the focus is on a project or activity or goal instead of on each other, people get to know each other gradually and naturally.
Participate fully. Share your time and your talents. You’ll find this much easier to manage than situations where you are expected to interact one on one. As you get more comfortable with people in the group, it won’t feel like such a risk to start opening up.
Please don’t give up if the first try doesn’t work. You have some experimenting to do to find your natural group. You’ve missed out on doing this for almost your entire high school career. I hope you will commit to using your senior year to stretch out and explore relationships with other girls. This will prepare you for getting to know people in college or the job you have ahead.
I wish you well.