My best friend won’t communicate or forgive

I need to get some advice on female friendship dynamics. I have a wonderful friend (I’ll call her B) whom I love with all my heart—we met in college and have been best friends ever since. However, I worry about her a lot. B has trouble communicating with her other friends and when something upsets her, she cuts them off completely out of her life. For example, we have a mutual friend who posted a photo of B just out of fun without getting her permission first. When she realized that it hurt B’s feelings, she apologized and immediately took it down. Now B is so upset. She won’t forgive her, even for something so tiny, and has cut her off. This is a pattern that I have witnessed over and over. B and I remain close friends but I watch other friends get hurt over and over by her bitterness and unwilling to communicate or forgive. Surely she can’t expect that all friendships are 100% perfect. There’s gonna be things that people need to work out.

I just don’t know what to do because I hate to see B isolate herself and I hate to see my other friends hurt by her. As B’s best friend I can’t pick sides and turn my back on her. What am I to do?