In such case,"私は、彼女にふさわしくない。"is pretty good. "AはBにふさわしくない"means"Ａ is unworthy of B" here. I think we can also say "私は、彼女に釣り合わない。"／"私たちは釣り合わない。"

A literal translation would be 「私は、彼女に十分ではない」. But 「私は、彼女にふさわしくない」 is more natural expression in Japanese.

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