I'm so sorry to hear this but like the previous writer has said the choice is very much out of your hands.
Counselling would be a good idea. If she leaves you to live her life her way or with this woman then a counsellor will give you the support you need to deal with the pain, hurt, loss and anger.
This is so sad. I very much hope that you can get your marriage back on track-but I think having wrestled with it and finally telling you about it-it sounds like she is making her mind up to go her own way.