Dee,
"Transitioning will be a permanent change - there is no going back to just being me."
Indeed, telling others that you're trans — intentionally or otherwise — isn't something you can take back. But, there are things you can do to test the water before coming out. It might help you to give yourself some patience. Would you like some ideas/thoughts on that?
" I spend most of my time in bed at the moment trying unsuccessfully to imagine what my life would be like if I chose to transition to female. Trying to work out a kind of pro/con scenario about my family & friends, my work and which colleagues would disown me and what direction my life would take in general."
I know what you mean and I felt that way too. I suggest that the first step is to accept that we can't control other people. They may be supportive and loving, unsupportive, or somewhere in between. It's your life, and up to you to decide what you need to do/be to be authentic to yourself.
"Am I fighting it just because I am transphobic on some level?"
Answer: YES! Remember that "phobia" = "fear" and you'd be crazy not to be fearful. It's what our amygdala does for us. It tries to protect us from the scary monsters and things like that. It's perfectly normal to feel these fears, all of them.
"I do not want to transition, but I Need to be authentic and honest with myself and the people around me.. If I understand the definitions correctly I have more social and emotional dysphoria than physical dysphoria - it is more like I experience euphoria when I physically look like how I mentally feel."
I didn't want to transition either. And even as I proceeded I didn't expect/anticipate what aspects of transition applied to me. I think you have to determine that, slowly step-by-step, by yourself with a therapist (ideally).
I also know what you mean about the euphoria. I certainly felt that way too. These days it's less and less. I just am another (somewhat older) woman in the world, living her life. I like getting dressed in something nice that feels good, and hanging out with friends. But I also like going hiking, cooking, and playing with my cat. I also do woodworking, gardening, watch movies.
I can't tell you what you should/shouldn't do. I hope that through small steps before you come out publicly that you can experience those steps — scary as they may be — and discover that yes, it feels right. You have to allow a little time for the euphoria to dissipate to know your true feelings. And then, cross another bridge, and another...