You are absolutely going about it the right way. You have to know about yourself before you can explain it to others. Jotting down your thoughts and feelings is an excellent idea.
When or if you tell her there will be many questions. The first two are going to be are you gay and do you want to be a woman. Followed by a barrage of many other questions.
I recently came out to my fiancé and it has been hell on our relationship. Everyone is different and everyone will react differently. There’s no way to tell what will happen. It is an atom bomb being dropped in someone’s lap. Stay true to yourself. Know the pain can be excruciating and accept that fact of it. Try to keep your thoughts organized and try to talk in a calm manner. I wish you luck as this is one of the hardest things I’ve ever done.