Thank you DingedHeart.
She was blindsided for certain.
We discussed therapy last night. She has a psych background and isn't open to the idea. For her any attempts to resolve this difference means that she will lose me despite my assurances to the contrary. That's what got her spiraling on the poly reading material as it discusses ways to work through it.
The sad thing is we have always had a good marriage but this psychological difference is too vast. It would have been easier had I admitted an affair or 'she meant nothing to me', but telling her I want an emotional, romantic relationship outside of our marriage is too much to ask.
I feel selfish for the damage I have caused and while I know I am poly I don't think she can ever feel safe with those feelings.