Fact: You don't have to be a flaming extrovert to be confident. You don't even have to be, or to have been, a good student.
Believe me, getting good grades does not correlate with confidence. It could even be true that people who graduate from college "Thank the Laudy" instead "Magna Cum Laude" are are more confident in themselves. When you get out of school, most people could NOT care less about your grades. And that's the way it should be.
I get a real kick out of working with certain clients who, like me, were excellent students. You probably know by now that getting good grades does not even correlate to being smart.
I enjoy so much seeing them blossom into authentically confident people by drawing on their inner resources they forgot they possessed.
Getting good grades is mostly a matter of pleasing the teacher. That's why girls tend to be better students in school, and why boys tend to do better in business. (Sorry if that offends anyone; that's the reality I see in the classroom and out.)
Remember when you were six and if someone asked you if you could draw, dance, sing, or tell stories, your answer would something like, "Duh....of course I can."
Getting your confidence back is something like remembering that feeling when you were six and your only career challenge was deciding what you wanted to do with yourself. That confidence is still in you, just gotta coax it to surface again.