This is a hard one in deed and I am sorry to hear of your struggle. I know it is hard and you got to make it right for everyone involved, but sometimes you can't please everybody and you have to do what you feel is right in your heart.
If you already told her that you intend on joining your 1st wife, then you need to sit down and just discuss it. You also have to keep in mind that she may be feeling like she is 2nd best. It would be a good thing if you make her realize just how special and important she is in your life. Insecurity does not surprise me. I understand her view as well.
Whatever you do, best of luck to you.
