It will be hard for her to see your pain. From what you say she feels guilty too. Take some comfort from the fact that your W has told you about this herself - it shows she has a measure of trust in you and I also think it shows a willingness to want to work on the M and improve it. All of those are positives.
I know it hurts really badly and there seem to be no excuses for what she did, but you can see why she was dissatisfied and unhappy in the M when this happened. Try and look to the future. Learn from this experience. Gradually you will move through the range of emotions you are having and settle down.
Just remember two wrongs don't make a right so don't go down the revenge route - it will only make things worse.
Saffie me 46 H 46 M in 1986 D20,D18,S16,D13 H's A 01/05 to 07/06 H recommitted to M 07/06 renewed vows 09/06 Going from strength to strength