Thanks to all the responses so far! Much appreciated!!
A brief bit more background... We started our poly journey 6 years ago when I fell in love with another woman. It was totally unexpected for me and my wife! I was honest with her about it but we were in a religious setting which made it even more difficult for my wife to accept. She did try, but it became too overwhelming for her and so the relationship had to end. This was very painful for both of us, which in a nutshell explains the pain I mentioned originally.
Six years later we have talked and talked and reached a place where we both agree that poly is something we want to try. So if we are not ready to try poly now, I don't think we ever will be.
So it so happens that my wife is the first one to find "success" in terms of meeting someone she has fallen in love with. This is difficult for me on one level because I am constantly reminded of what was not possible for me. I see her joy and the excitement and I grieve what I have lost.
But the greatest pain has been seeing how open she is with him. We have been married for 20 years and there has always been a hint of holding back from my wife. This new relationship shows this up even more. I have always hoped that she would be more open with me, and it hurts to see that she can be with someone else, and that someone else is getting immediately from her something I have been hoping for for over 20 years.
However, some of the comments here have been really helpful. It is important to realise that every relationship is unique and that she will relate in different ways. Also it is important for me to recognise that it will be easier in many ways for her to open up to someone new. Rather than seeing it as a deficiency in me or a negative thing, it can actually be really positive and my hope is that she will learn to open up and in this process work through the issues that hold her back from me. She certainly wants to do this. So this new relationship could very well be the catalyst that helps to bring about a deeper openness in her that I have always longed for...
In the meantime it still does hurt to see it happening with someone else... Reminder to self - stay patient and remain focused on the bigger picture...