Friends with benefits

So the person I was friends with benefits with happens to be messing around with his sisters neighbor, his sister is my best friend and he is almost 25 and the neighbor just graduated high school this month , I’m just really hurt wanted to vent, I cut him off completely, he doesn’t know the actual reason but dealing with pain for a year and a half of wanting to be with him and him not wanting to date again for his reasons has finally gotten to me, I want to message him but I kno I shouldn’t