Commonality does not make negative emotions less terrible. A support group doesn't make a condition more or less common. A support group makes you less alone, which is another way of saying you feel your emotions are validated and youre relationship to your emotion changes in that group. Some people need a support group to change their relationship to an emotion.
> Commonality does not make negative emotions less terrible.
There is a saying in Italy: "a shared misfortune is half a joy".
There's a recognized therapeutic value in literature. Reading novels doesn't validate your emotions- reading is unidirectional, there is nobody telling you "I understand you" from the pages of a novel. But you get in contact with characters who suffer, confront problems, overcome them or succumb to them. You can see their inner life, and recognize a commonality of human experience.
Conversely, social media are notorious for making people depressed: it can be as unidirectional as reading, but since people tend to show only the positive aspects of their life, it can make you feel your own life is especially bad.
I think the keyword here is "common". Negative experiences become especially bad when they're perceived as unique, extraordinary. Framing them as normal facts of life helps dealing with them (think of support groups: they reframe personal experiences that might feel uniquely terrible into normal experiences inside the group).