You seem like you really care about her and I'm sure you do. But I'm going to give you the straight dope: if you care about her as much as you say you do, you need to tell her now. Today. No more waiting. You seem like you are in a terrible despair and everyday that goes by with you keeping secrets is only going to hurt you and her that much more in the end. I'm sorry this is happening but you need to come clean. If she leaves you, she leaves you. I know that sucks. But know what sucks more? day after day in a loveless, fictional partneship. I say fictional becuase she does not know the real you. And BTW, she might truly have no idea. Don't assume she does because you are not manly or whatever. Jeez, that's such a stupid thing to say and kinda immature. Maybe it's time to start a new, beautiful life living as you.
BUT, be sure you are certain you are going to transition. I have no idea what the detransition rate is, but there are a lot of people who just drop off this site and never return. And they drop off right after they talk about transition was a mistake. So be sure. This is not for everyone.
I have never been married but I was engaged and my ex-fiancee knew from the get go that I was a transsexual. In fact, the entire reason she dated me was because she said she was sick of men and then she met me and it was perfect. But she changed. And said she could no longer be with someone like me anymore. At the end, she made constant comments about my shoe size, my hands, my boobs, and everything else about me. And not in a good way. (my body is pretty feminine without HRT and I wear a size five and half in men's and have tiny, tiny hands and she could wrestle me to the ground.) There was a point when she told everyone I was a woman and her girlfriend but I had a birth defect. But when I got serious about HRT, and then told her it would be great if I had a uterus, the engagement collapsed. But in the end it was for the best. Now, I have a super hot, loving boyfriend and she has a nice husband. We never talk. So things will get better. I know the hurt but you must go thru it.