Having been on both ends of the scenario and see the aftermath, your best walking away and finding peace. You however need to forgive her, not for her but for yourself, this will allow you to move on and find what you truly deserve in life.
If you stay with her, trust me, she will cheat on you again, and unless you have the strength of a thousand men, you will be slowly broken down.
Beside, she just was not that into you