Josh, being afraid of initiating change, especially such a profound change, is always scary. It's normal and natural to feel the way you do. You fears don't indicate a lack of commitment to transition, just a normal and natural response to a stressful situation (or at least perceived as such), especially at your age. I applaud your courage (yes, COURAGE) in making the decision to see your doctor. When I was you age, there was no WAY I could have done that. Of course, it was a different world then. Even now, I went to my doctor last July to ask for a referral to the Gender Identity Clinic. He knows me as a 54-year-old man, married over 30 years, he treats my wife and kids also, and has known me for over 10 years. As me how nervous I was to tell him about this!
But doctors are professionals, and even if they don't have specific training about transsexualism, they do have training about how to treat patients, and they know how to be professional in such a situation. But be prepared - your doctor may not know any good therapists in your area, and may not be well connected to trans resources. That is something YOU may need to find on your own. Sad, but such is the way things often are. But don't be afraid to reach out, you'll be amazed at the resources you'll find if you do.