Pretend she's lied to you for weeks but has had HER ex-husband living with her while he gets back on his feet, and he's the father of HER child. She assured you that they never had sex, but she's got a very small apartment. What happens after he gets back on his feet? And she's loved him for a lot longer than she's loved you? And he's her first great love, even though she realizes now that he was a really bad choice, but it really was good at first! (She won't tell you about the good parts, but you can sense that there must have been some, because she's not that stupid that she'd fall in love with a guy who's ALL BAD. Or maybe he just turned from really good to really bad when his soul got sucked away by powerful drugs like coke or meth. What if he gets clean and becomes a better man than he ever was before?
I'm suggesting that you get inside of her experience--with her wild-'n-crazy emotions completely turning her head around in a whirlpool like a toilet going down the drain--and how can she trust your hand to lift her out when you've been lying to her all this time, to her face?
I'm not saying YOU DID ANYTHING WRONG, since you must have thought you'd get away with it and save both of you the great pain that telling her could have catalyzed. I'm saying that HER EXPERIENCE WAS AND IS NOT AT ALL LIKE YOURS, AND THAT'S WHAT YOU'VE GOT TO FEEL if you want to reach out to her in a way that shows you care about what your actions have done to her. THAT'S THE PREREQUISITE OF REMORSE, and remorse is the best way of starting to make amends, and of guaranteeing that you'll never do that again. DON'T MINIMIZE WHAT YOU'VE DONE as you've imagined it for your own convenience, BECAUSE THAT'S NOT WHAT SHE'S GONE THROUGH AND IS STILL FEELING. [That's why I wrote up the reversal of her experience in more dramatic terms than you'll insist was actually the case in your mistake. But EMOTIONS ARE DRAMATIC, especially when the person who's having them is in the dark about the situation, and feels therefore HELPLESS.
You won't want to do this with your heart in it, because IT FEELS AWFUL. I've been there--though not for the same hurtful actions. IT FEELS HELPLESS AND ASHAMED. But that's what you need to show her that you're feeling, because you're really feeling it.