I don’t know where you get the idea that most people your age have had sex. A recent study of Romanian teens showed that only about 20% have had their first experience with sex before age 20. Teens aren’t always honest about their own experience. They often want to seem more experienced, more sophisticated and more knowledgeable than they are in order to “fit in.” Wouldn’t it be ironic if all the people you know are just trying to impress each other and none have actually done any of the things they talk about to fit in with other people who were also telling tales?
Of more concern to me than your sexual inexperience is your discomfort around other people. With a therapist’s guidance and support, you can learn the social skills you need and to feel more confident socially.
As for your worries about therapists: Therapists aren’t interested in judging. They are interested in helping. I doubt very much that a therapist will make you feel ashamed. (If one does, leave immediately and find someone else.)
As for a way to start: Just bring your letter and this response with you and ask the therapist to read it as a way to start your first session.
I wish you well.