I am basically in your shoes, and I get the heartache from missing the other person who has a piece of your heart. Its been a month and a half since I was allowed to talk to mine. I have "come to my senses" or stopped loving him. I have stopped crying most days though.
I would imagine that your wife is still reeling from the fact that you have been cheating and lieing for 3 years. It would be really hard to turn around and say "Run along and be with her too" if she doesn't trust you, and right now, she most likely doesn't. But she didn't leave you which after knowing you had an ongoing affair is a great start.
What about seeing a counselor? I hate to see someone throw away such a long marriage where it seems you both still love each other. Its not like you asked for a new pair of shoes and she flipped... You asked for a girlfriend. A completely foreign concept to her, something that seems like it would make her feel foolish if she accepted (from her comments about no one would do it). Backing her into a corner probably won't work.
However, I am NOT saying its ok to just be broken hearted and feel like you are living a lie. Realizing you are a poly person can be a life changing thing. You may not be able to stay in a mono marriage and have self esteem left. But she may be able to come to a point of acceptance with time and help.
Idk, I'm rambling here. I know my life hasn't gotten fixed yet and there are many days I just want to call mine up and say "I love you and miss you SO much!!!". But to do so would NOT work because itd set back all progress we've made so far.
None of this has been easy, but I'm worth fighting for, and so is my husband, so I can't just walk away, I have to work until. No set timeline, just until it works, somehow, for both of us. Sadly, until seems a long ways off.
Hope this doesn't sound preachy, just wanted to remind you that I think you would mourn the loss of your wife if you were to walk away. It would likely be something you regret if you don't try to work things out first. It may not work, but I can't imagine ever feeling "I wish I never tried to fix problems with my wife of 20+ years"