Take a breath. It's not as tragic as it seems. You have not cheated and are doing the best you can to ease her into what is going on for you it seems. It will take time, and you have time. This is all new to her. You have been living with this for a long time and she is just hearing of it now. Keep strong and committed to the course. You are being honest and vulnerable and respectful to her. That is all you can do... you are doing great. Just love her with all you've got right now.
Tell her that the woman at your office has caught your eye, but that was merely an avenue to talking about it, not something to necessarily pursue. It's okay to be unsure what the future holds in that department. Just as you said here. That is what I would do anyway. Better to get it all out than keep some for later. Besides she asked, and you then lied.
Confessing and apologizing would be good at this point I think, while you can fall back on being confused and not wanting to hurt her. Leave it longer and she WILL think she can't trust you. Either that or I would really vow to not see this other woman again in terms of how you have been. And stick to it!!! If you decide this latter route, it needs to be done with her I think. You need to believe that. No attempts to add her later. If you pursue her later, its likely your wife will find out that the two of your were in that attraction energy before she knew about it. She will definitely struggle to trust you then!.. if you want something to come of it later, then don't lie and fess up now..that's my thoughts anyway.
I totally get that she would think its her and start doing things to make you love her more. That is her self esteem being tested. It's totally natural and totally acceptable. You know that she is loved by you no matter what, so keep telling her and creating plans to make sure she knows that. Keep reminding her that your love for her is unmoved by this new information to her. You have loved her all this time and have known for a long time that you have it in you to love others... why would it make a difference now to alter herself so you will love her more.
A therapist might be helpful at some point, but really, you are talking, being open, honest (as much as you can and in baby steps), working towards a future... that is all a therapist would have you do. This work is HARD. Relationships of any kind are HARD. A therapist will not necessarily make it easier or make it go away... if you need focus at some point they can help with that, but you still have to do the work.
I would suggest giving her the whole can of worms and getting her on line here and elsewhere. Start learning and education yourself together. I agree with the book idea. I wouldn't suggest "the ethical slut" just yet, maybe "opening up," or "love without limits?" There is a whole thread in the stickies on books... you could look there.
At the very least you might want to do a search here for people that are in her position... The new to poly section is filled with similar stories of partners coming out as poly.
Keep at it. Things are hard it seems, but you are doing great so far.