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It wasn't very thoughtful of you to forget your promise.
Literal
Promise [obj-を] forget [quotative-とは] very thoughtful [was-not-ではなかった]-[polite-です]-[confirm-ね].
~とは here is the evaluative/exclamatory quotative — '(the fact) that you X' — used to frame the preceding clause as the basis for judgment. The past negative 慎重ではなかった ('was not thoughtful') + ですね softens the criticism with a shared-confirmation particle. A subtle scolding that feels more like regret than anger.