Welcome to the site.
We actually have an entire category devoted to coming out. It is never easy. As Tessa James said often it is hardest to accept in ourselves. However, it sounds like you are already there.
I would ask if you are currently in therapy? If so, this could(should?) be the focus on a number of sessions. I guess my next question would be how settled on who/where you fit in, are you? Does that need to be focused on before you can talk about it? I know when I first started therapy, I thought I was a cross dresser. But it was not enough. I finally admitted that I am MTF transgender. From there I could/am exploring how far I have to go to be comfortable in life.
Unless your SO somehow guesses, you will probably have to initiate the conversation. It seems that absolute honesty is what helps relationships weather tough times. Including honestly saying you don't know or are unsure about something. Be prepared and imagine any question they could ask you. You have had a long time to think about this. When you tell them, it will all be new. Be prepared to answer questions. Don't overwhelm in the initial explanation. Be prepared to answer the same question a couple of times. Give some space and breathe.
That was the advice I was given and it helped with some of the people I told. I am not done telling(they claim that once you start it is an on going thing for quite a while).
Good luck. Come back with more questions as needed.
I also want to share some links with you. They are mostly welcome information and the rules that govern the site. If you have not had a chance to look through them, please take a moment:
Things that you should read
Once again, welcome to Susan's. Look around, ask questions and join in.
With warmth,
Joanna