Why is my cheating boyfriend's mistress gloating at me?

My boyfriend of a year cheated on me with this girl whom I know and used to be friends with. After they cheated, I found out because she sent me a text from an unknown number gloating in detail about all the things they did together in the bedroom.

She told me I'm ugly and that's why he couldn't stay faithful. That she is better than me. Why would the mistress or affair partner gloat? Haven't they both caused enough havoc and pain?

Why is my cheating boyfriend's mistress gloating at me?
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