My best friend keeps canceling plans at the last minute

Dear Annie: My best friend has become impossible to make plans with. She says yes to everything, then cancels at the last minute with a vague excuse. Sometimes she says she is “too tired,” other times she says work got crazy, and once she forgot completely until I texted from the restaurant asking if she was on her way. The worst was when I saw on social media that she had gone out with another group the same night she canceled on me because she “needed a quiet night.”