I feel like my heart is drowning in vengeance after my girlfriend of 9 yrs cheated on me and is now dating that same guy

How do i deal with this? Its just not fair, known her since i was a child and we made future plans and all and she just stomped all over my soul. She was even starting to go Church weekly. I cant survive knowing she built happiness on my suffering, and now all i feel is vengeance… i pray and feel temporary relief, but it just eats me up during the whole day knowing someone you once loved could hurt you so badly.