Just found out my best friend kissed my bf!

I started dating my boyfriend 5 months ago he told me he loves me and all that bs. Now I find out my best friend that I've know for 16 years made out with him really I more hurt by her than him. 5 months if dating isn't that long. I broke up with him now I have no idea what to do with her. She's like a sister to me but feel like I got stabbed in the back what should I do? Help!