Cis heterosexual lady checking in.
I reacted very badly. When I first found out I cried every night for months. I had a constant headache. I started binge eating (still am...actually.) I remember one night specifically after my kids went to bed I wrapped myself up in her old male button up shirt and cried myself to sleep. Even writing about that night makes me want to cry again...lol. Anyway. I hit an all time low and was that way for a long time. Now I am at the point where I am exhausted from always being tired and am just always angry.
The sooner you can tell her, the better. My partner has been my best friend for 10 years before we were romantically involved or anything (we've been married for 3) and I did not expect this coming. I actually felt (and still do, though not as much) very betrayed that she didn't tell me sooner...We had always been so open with each other...she knows all of my deepest darkest secrets. Sometimes I feel like I don't even know her anymore or feel silly because I put so much trust in someone but the same was not returned.
Be as open, honest and upfront about it as you possibly can. Don't beat around the bush...be completely straightforward. Her feelings will be hurt, she will mourn, she will be confused...but she deserves the truth, and all of it. Half truths sting a million times more because eventually they are found out. I feel like my partner went back and forth a lot and it made things even harder on me when she went through the moments of doubt or saying " i wont go farther than this" or " idk if i actually want to go on hormones" when she really did because it made me feel like things would go back to how they were, and then she would change her mind again and I would mentally be back to square one.
Give her time to let her digest all of it. If she decides she wants to see if she can make it work then that is up to her. If she decides she cant, you have no right to guilt her. please for the love of the lords of cobalt don't ever ask her anything like "if i lost my penis in a car accident would you stop loving me????" or "were you only with me because I had man parts?" It doesn't help, it makes everything worse, and it will hurt her more.
If she decides to stay, don't make it all about you. My partner was so obsessed with herself and her transition that she forgot about me. It has caused a big rift between us. I know she is sorry but I have a hard time stomaching how you could treat someone you love like that.
If you want to stay together through your transition then you have to go through it together. She is transitioning right along with you in a different way, don't force her to do anything she isn't comfortable doing yet because you are impatient. You have known you are trans for years, she has just found out.
When she first came out my partner showed up to my mom's house with nail polish, eye makeup and jewelry on and of course my conservative mother went ballistic on ME. My partner went to my work several times in full girl mode when she came out to me before I had even told any of my coworkers. It made work, my one place away from the transition that was taking over my whole life, very, very awkward and uncomfortable. I was forced to tell everyone that my husband was now my wife before I even had time to properly stomach it.
Hope for the best but prepare for the worst. I truly hope everything works out well for you. Good luck