I would forgive her if she asked for it and was truely remorseful and repentent. Forgiveness is a gift you give to yourself. Peter asked Jesus how many times should I forgive my brother 7 times? Jesus replied not 7 but 7 times 70.
but then there is also an old verse about, dont cast pearls before swine, they will only trample them and then attack you.
So that is why I say only if she is repentent.
I'm struggling with a similar issue, dont know if shes cheating but have a strong suspision. I dont want to know to tell you the truth, just left and got away from her. if she comes back ------great till then I'm moving on and meeting some very nice women!