I am so sorry for what you are going through revisiting the loss of your other love. I daresay that due to circumstances you probably did not have the opportunity to grieve the loss properly, and now you are finally free to do so.
As the "other love" who was recently booted out of the triad myself and for the same reasons you cite, I understood going in that the husband would have to stand by his wife if our attempt failed. I grieve the loss, but don't really blame anyone. Given your other love's more fragile state, I don't know if she was able to be as logical I am, but on some level she must realize the horrible position you were in and that your forced to choose.
Of course, you don't want to screw up her current life, and given the circumstances, she may be totally against the concept of polyamory now, but you may be able to induce a little healing if she is open to communication with you. Is it possible to explain what has happened in your life (your wife's move to polyamory), and express to her your feelings on her loss, something you probably were not able to do properly given that you had to be the strong one at that time. It probably won't bring her back to you, but perhaps it can help heal the old pain between you.
If she is doing well in her new life, you may feel some joy and relief that her new circumstances are okay. It won't keep you from missing her, but it may assuage the guilt.
Of course, it's possible she may want nothing to do with you. And then you know you have to put it to rest once and for all, and move forward.
(I have a post on the subject of being the "other love", that is still awaiting approval from the moderators unless it was lost in cyberspace yesterday).
Again, I am sorry.