First, I'd just like to say"Hi!", and give you an unofficial welcome here. One of the moderators will be along soon to give you some starting info on the site. This is a great place to help you in getting started with your therapy, and with any other issues that might come along. Friendly and helpful support here!
Scheduling that therapy session is the right move. In your first session, the therapist will guide you a bit to tell them about yourself and what issues you might have. Definitely tell them about that sensation, of "part of me has been trying to claw it's way out of me". Let them know you are questioning your gender. They should ask additional questions about that, and guide you in clarifying just what is going on.
Just try to relax and talk about what is bothering you. Therapists are non-judgmental folks there to help you help yourself.
Good luck, and I hope your first session is productive for you.
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