Should I be mad at my best friend for dating my crush?

My best friend Sandra knows that I have always had a crush on Emma, our childhood friend. Emma likes me too but it is obvious he keeps holding back for reasons best known to him.

I just found out last week that Sandra and Emma have started dating. I heard the news from my crush (Emma) and I was completely heartbroken that Sandra couldn’t even tell me herself.

I don’t know what she told Emma but when I confronted her, Emma overheard us and told me not be mad at them and pleaded that I should let it go. Honestly, I am so mad that I just want to stop being friends with both of them. Please I need advice?