At 19 when Anne was persuaded to break her engagemnt, she gains the strength to do this because;

Ch 4
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;

7/8 years later the same incidence arouses a different sentiment regarding the break up.

She was persuaded, that under every disadvantage of disapprobation at home, and every anxiety attending his profession, all their probable fears, delays and disappointments, she should yet have been a happier woman in maintaining the engagement than she had been in the sacrifice of it; and this, she fully believed, had the usual share, had even more than a usual share of all such solicitudes and suspense been theirs, without reference to the actual results of their case,

It's interesting to note this difference in the way of her thinking.

She is now able to think of herself and her happiness.
I think this was possible because;

time had softened down much, perhaps nearly all of peculiar attachment to him,

Anne is able to think rationally both the times, so she is a rational person.
It gives one consolation to think that Anne's rational thought can also include her own welfare when emotion doesn't interfere - this I think is necessary for her to deal with the present situation.

Of course the word 'imagine' gives one the impression that she was looking for a reason.