I agree with everyone else, you have to tell her of course. It would be a terrible thing not to.
You also have to tell her for your own sake, not just hers. It may look like worst case is you tell her and break up, but I expect its actually if you don't and keep going. Imagine 5, 10, 20 years from now and you have children, and hiding this secret has poisoned your marriage and possibly your relationship with your kids as well, and its all your fault. And eventually you can't take it any more, like so many can't, and then she finds out. The guilt would have to be appalling, and you'd only escape it if you were a sociopath, which you're not or you wouldn't be asking this question.
I've got kids, and its been rough, but at least I didn't know I was trans. I still feel its all my fault. I know in my head that's not really true, and it helps somewhat. I'm can't imagine how I'd feel if I'd known and lied. I'm not sure I'd survive it.
You might also be fortunate and build a better relationship with her, or failing that someone else. Even ignoring the worst case, being with someone who loves the real you would be a wonderful thing to have.