First off, I am in favor of full disclosure as soon as it's clear to you who you are. I think the time of discovery that most of us seem to go through is a grey area and I know we are dealing with our own demons during that time and might not be in a good position to share, even though we might want to, I know I was. I guess I would ask a couple of questions, forgive me if you've answered these in another post but you needn't tell me in any case. You're sure enough of some things to go ahead with an orchi. That's a fair amount of commitment. Orchi not with standing, does your Mother know that you are that sure about your GD and future desires, how much have you discussed with her in general?
If your Mom knows a lot about where this is going, is she supportive? If so, you might take a leap of faith and tell her and hope she will continue to be on your side. She may want to know why she wasn't consulted in process, and even feel slighted by your not getting her in the loop, that's a real danger. I know first hand that some will play along, hoping you'll just get this out of your system and as they see you move more and more into the reality of some sort of transition, will fold. Relationships are so incredibly complex, only you and those in your world can even begin to know the complexities you are dealing with, so this is just a heads up.
On the other hand, is there any reason your Mother should ever know about your anatomy? Unless you are worried about someone else telling her, likely to cause a firestorm, how would she ever know? If she isn't supportive and you need to proceed, consider just doing you and keep your mouth shut. Just things to think about. Toni