Fearing you might not be meant for someone can be very hard. If you get mad at your Mom, your Sister, you go away for a while, but they are still family. You know on your deathbed if they live, they will be at your side.
But a mate, especially in monogamous relationships, when you let go; when you know you have to let go, you are torn; torn between wanting to move onto the kind of person you really want, and being hurt thinking of possibly letting go of one who has become the closest kind of family to you. They may find a new mate, and you could even lose them as a friend someday. That thought keeps some even bad relationships alive. I'm not saying yours is (could become a wonderful new way of life for you), but lets evaluate to answer that question.
Oath breaking should never occur in any kind of relationship that is considered mutually beneficial. Without perfect trust, a relationship is ....well ownership. If I stay with a man who I think will screw me over, I know this is not good for me. I know it will cause me stress, and my cortisol levels will rise. I will gain weight, have panic attacks. To stay is like allowing yourself to be hit and abused....just from the inside where it is far more dangerous than a black eye. Seriously. Too many people don't think of it that way.
So is she the woman you want or not? Well, she's proven she must have someone other than you, so you're s*&t out of luck if you want to change her. She won't. Chances are even if she tells you she will, she won't. But is she a cheater....or is she poly. Could you trust her in other ways before this?
More important to ask first though, are YOU poly?
Ask yourself this: Can you imagine an instance right now where your wife could be kissing a man, and you could at least call that man a friend, and love him for the happiness he is bringing your wife?
Is there any guy out there, you could tolerate this with. Imagine for a second you could even hand pick him where he is someone you'd love to hang out with...but he kisses your wife. Will you ever allow this without being sick?
Once you know the answer to this, you know what you need to do?
You either leave her and move on to a girl you can trust not to betray your way of life that you have every right to, and don't let your wife tell you any differently if this isn't what you want. Live life to it's fullest.
Or, you now need to establish an all new kind of trust. Let her know she betrayed you, but that because you understand she wants more love in her life, and not less of yours, you will give her a second chance. Throw away the past. Let it go. Both of you dump your emotional ammunition and be as two completely new people, agreeing to bring up the past only when they are happy memories, or significant, but never ever ever to play the game of "YOu said I did this? Well what about what you did on a Wednesday". If you are that kind of couple, poly will rip apart your world.
I promise you!
Fall in love again with her through this new understanding. Get to know each other. Talk about the kind of mates you both could tolerate them being with, and what your boundaries would be (If you want to know some of the ones used in my life, and I can be of help in sharing, PM me sometime).
When you come to a happy middle, set those rules in stone, unless you both wish to change the rule together. Never ever go off to break a relationship rule without prior consultation. Never! That's a cheater and not a loving poly relationship. Best to give as few rules as possible, because one thing about poly is the enjoyment of freedom, yet we all have our limits. It doesn't make two people bad if they decide they can't live up to each other's rules. It's only bad when you waste your lives staying together bickering about it.
I wish you both the best of luck.
Perfect trust, or not at all. Let there be nothing else, or you are dying instead of living.