It is a big number, but you've been with her now for 3 years. You've made a decision to stay with her 2 years ago when you found this out meaning you accepted this fact and you accepted her as is. Now you have to take responsibility for your decision and just take her as she is. She is not sleeping with any one now, so just let it go. Be happy that this is (apparently) the only major issue in your relationship. A lot of others are not as lucky as you.
Don't cry, don't regret and don't blame
Weak find the whip, willing find freedom
Towards the sun, carry your name
In warm hands you are given
Ask the wind for the way
Uncertainty's gone, your path will unravel
Accept all as it is and do not blame
God or the Devil
~Born to Live - Mavrik~