I think KitKat's advice is sound.
Honesty will always reap it's own reward in good time so you might consider telling her you sought advice and print off the pages of this thread to show her or log into the site and let her have a browse for herself.
HOWEVER, only you can judge the best way to approach telling her what you have done in her absence. I don't know how much honesty there is in you're relationship but if there has been little before then this may be a start in earning some respect from her for your integrity She may not like what you have done but it should give her food for thought. I'd suggest though that you make it quite clear to her that you respect her feelings about it and endeavour to impress on her that your loyalty and committment is to her, and her alone and that nothing is more important than your relationship.
If she walks out on you then this is just the straw that broke the camel's back and sooner or later things would have gone tits up anyway.
It really is so very hard to know what you should do because no-one here has the slightest knowledge of, and insight into, your relationship.
Just my thoughts but you have asked.
Hope this helps. Good luck!
ES