Okay okay so. I have a theory about Anne and Wentworth's initial engagement about Persuasion, and it drives me very nearly feral. Especially when you tie it to later conversations she has with Benwick and Harville.
But it was not a merely selfish caution, under which she acted, in putting an end to it. 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.
The special legal privilege a woman had in an engagement, which her fiance technically didn't, was the ability to back out.
(Yes, it was not without social consequences; and yes, if her fiance backed out, she'd have to actually publicly take him to court for breach of promise to receive compensation, which many girls (and more importantly, their families) were not eager to do.)
But imagine Lady Russell telling Anne: He's a sailor. If you stay engaged to him, the Admiralty could order him to sail to the end of the earth and map the coast of Antarctica if they wanted. He could be sent anywhere. He could be marooned, shipwrecked, deserted, made a prisoner of war, for years. Imagine him, in that distant situation, meeting some other woman and falling deeply in love with her. Wanting to marry her. And not being able to... because of you.
You would blight that happiness. You would chain him to an uncertain promise forever. You would drag him away from that new life, because of some silly, foolish, stupid decision you made when you were nineteen. If he still loves you, let him come back and say it then; but don't force him to come back if he doesn't.
(Given her family, she's probably very used to the sensation of being offered the outward forms of belonging and affection and knowing that she would be an idiot to hope for anything solid underneath.)
So then when she breaks the news to Frederick, the actual reason down at the core of it is, "You're probably going to forget about me and fall in love with someone else anyway, so we're just making it all easier on ourselves." Which he is wildly unimpressed by.
