My best friend went behind my back with my ex, twice. Do i forgive her?

She no best friend of yours. In fact, she's poison. If she doesn't care about your feelings after knowing you for 10 years, she never will. Kick your ex bf into touch too. You need him like you need a hole in your head. I know it's hard for you to accept this now but believe me it's the best thing to do in the long term. The sooner these two are out of your life the quicker you'll move on an find a better life.