Do not think me unkind for such an exercise of my power...
Lady Susan would not have things any way other than hers. She has powers. She knows how to exert them on others, and she does not prevent herself the glory emanating from them no matter how ill-used they are. She does not care about anything or anyone other than herself, and she is ready to sacrifice anyone to fulfill her aims, which are either directed toward greed, glory, or feeding her pride.
...nor accuse me of instability…
It is amazing how she is able to twist everything to her advantage by giving well-formed reasons. She makes up her own version of events, even if there is not a drop of truth and she delivers her lies to her surroundings (Reginald, her sister-in-law...) with composure and confidence. She has an answer to every question, which makes Reginald’s doubts dissipate faster than they were formed.
…without first hearing my reasons.
Her composure and her ability to have an explanation to everything make Reginald believe her even if the truth is right in front of him, even if evidence is palpable. She trust too much her powers, believing that everything will go as planned; as if she was the master of puppets and everyone will fall into their place, fulfilling their roles. But fate had it otherwise since it is more powerful than her.
In short, when a person is always to deceive, it is impossible to be consistent.