Work on your confidence by yourself. "Love yourself before you love anyone else."
You do love yourself, enough anyways to seek a mate to satisfy your desires. You may not love everything about yourself at this exact point in time because of social stimuli and what the world/media/popular kids at school lead you to believe. Condition yourself to not care, not focus on it, let go, enjoy it, be in the moment, stay present, engage, and not worry about others opinions. Your lack of confidence is a direct result of you thinking one look or attitude is better than the other and then comparing yourself to said "ideal" image, or whatever it is you feel insecure about.
You are confident enough to get a girlfriend, an attractive one at that, so you're only worrying yourself more by thinking you're inadequate. Maybe you haven't had sex with her her? If you haven't, once that happens I bet some of your problems are alleviated! Stay focused!
Take some sticky notes and write in a big sharpie or marker in bold letters and stick them to your mirror or wall, wherever you will be forced to look at them every morning when you leave. Write good things, like: "I am strong. I am confident. I am handsome. I am worthy. I am happy! I am joyful! I have great hair! Great smile, handsome! My girl loves me. I love my girl (if you guys are at that point). I know that sounds cheesy, but positive affirmations really do help.
If the mind can conceive, the mind will achieve. If you begin having negative thoughts/patterns, you must release them and replace them with your normal happy ones, or new happy thoughts/images. Do not focus or dwell on the negative. Every day is a new day, another chance to grow, succeed, and achieve the mindset that you desire!
We're all human, all here on this earth together, living in the present, experiencing the same world, same problems, same feelings, and same pressure. Some people learn early on that they have to take control of their mindset and focus on being happy and confident, because let's face it, life can be hard and there are tons of things that can negatively impact your mood! Failing grade on paper, car breaks down, animal/pet dies, family/friend dies. Everything action has a reaction. Feeling the way you feel is completely normal and that's exactly why you have a girlfriend. At some point or another, she has felt the way that you do. You don't even have to explain to her, trust me she knows. Women, by nature, are extremely insecure in this day and age. Pressure to have "on fleek" eyebrows, makeup that looks amazing, a tiny waist with huge boobs and a giant ass, with big plump lips, that, also has to have like 5 different makeup/lip glosses on. So yes, your girlfriend has felt most definitely, undoubtedly not confident at times.
Let yourself be happy and try your hardest to make her happy. I promise you will eventually get out of this rut and realize you have a good girlfriend and are a great boyfriend to her, or any girl out there. Enjoy your youth buddy