I do believe that the text says she was only convinced to break the engagement as an act of self-sacrifice.

"Had she not imagined herself consulting his good, even more than her own, she could hardly have given him up. The belief of being prudent, and self-denying principally for his advantage, was her chief consolation, under the misery of a parting, a final parting; and every consolation was required, for she had to encounter all the additional pain of opinions, on his side, totally unconvinced and unbending, and of his feeling himself ill-used by so forced a relinquishment."

She would not have been able to "give him up", or actually gone through the face-to-face breakup, if she hadn't imagined that it was for his own good. "But it was not a merely selfish caution, under which she acted, in putting an end to it." She was not motivated by caution, but by the belief in his advantage. The final parting was miserably painful, but she held to her belief in order to stand firm and not give in to CW's powerful and emotional persuasions. I can't see that she thought up that argument later; I believe it was Lady Russell's most powerful and indefensible (to Anne) argument that weighed the scale against Anne's wishes.