I don't know if this will help or not, but I hope it might...
As you agonize over your impending appointment, you mentioned "flip-flopping"... It's possible that this is caused by attaching a huge amount of significance to the appointment. I just want to reassure you that while it *is* a big step, it is not a final step (or anywhere close). The first appointment with the therapist is a getting-to-know-you appointment, where you will learn about your therapist and they will learn about you. It is generally a free appointment, because you and your therapist need to see if you are a good match for each other. Can you trust her/him? Can s/he help you?
Beyond that, it is very unlikely that you will get any definite guidance from the first appointment. It has happened (rarely, and usually for us older transsexual people) that a therapist agreed with our assessment of ourselves and our need to transition (it happened for me, but I'm 39). And even that would not have happened for me, I think, if I hadn't made very plain that transitioning was something I looked at in a very positive light. Most gender therapists know how monumental transitioning is, and how scary/hard it can be to even think about. This is why it can take months or even years of therapy to a) determine the patient needs to transition and b) get them to a good place in their mind where the can transition safely and with confidence.
*hug*
You got this! Once you've been through the first session, I think you will be amazed at just how easy it was, and wonder why you were so worried.