Dear its not a trap Jayne set. Its a trap that you built. Many of us have been there. We build the trap for what we believe are all of the "right" reasons. You want to protect her, you love her, its easy for her not to know. Explain that to her. There is great advice in some of these posts especially Kaitlyn's. Ease her into this. Karen is also right. Expect to loose all trust in your relationship and never recover all of it back.
I know it sounds gloomy, but its a lot less gloomy than not telling her.