hey, i feel alot similar to you in many ways, i recently saw my doctor who has referred me to a gender identity clinic where hopefully i can see a therapist to help me understand everything. unfortunately as i live in the UK and its all nhs run, there is a really long waiting time :/
i still am pretty confused about it all and am not really sure what i want, and if i am even this at all, so i dont know.
I am really kinda socially isolated with not many friends, and i told my sister how i felt first who was really cool, and then told one of my only friends (although alot of homemade cider did give me courage).
I told my mother a couple weeks ago, and although she doesnt understand, and we sorta have an agreement not to talk about it, she still said she loves me and will support me no matter what i choose, even if she doesnt see it as me, or feels against it.
as to how i told her, i just sat down with her one night, and said i have something important to say, said how i was always feeling down, and always got into arguments quickly, or felt irritated because of it. in the end i didnt have the courage to say it straight up, but she said it and i just affirmed it.
as to keeping a journal, i think its a great idea, it really helped me understand what i feel, just by being able to read it, although i have been lazy recently and stopped writing in it, i really should get back to it. its so much better to write it down than have it rolling around in my head, making me feel insane and crazy, and stopping me from thinking about anything else. i say you should definatly start one, and in the end it really doesnt hurt to have a place where you can put everything you feel.
as to your therapist, im pretty sure they do not have to tell parents things due to the patient confidentiality stuff, at least in the UK anyway. as a minor it might be different, or maybe in a different country, but i think it would be covered by that anyway.