How do you deal with betrayal?

The friend whom I have been close friends with for 6 years, I feel betrayed by. I have an ex, and we broke up 4 1/2 months ago. I told her all about the break up and all the jerky things he has done. She told me that they started talking and she is starting to like him. It makes me so sick how she would do this. I don't even like my ex anymore, but the fact that she can do this after how much I've trusted her, makes me sick. She knows how I felt about him, he was my first and only boyfriend. How do I deal with betrayal?