A few months had seen the beginning and the end of their acquaintance; but not with a few months ended Anne's share of suffering from it. Her attachment and regrets had, for a long time, clouded every enjoyment of youth, and an early loss of bloom and spirits had been their lasting effect. (4.6)
Time is subjective – through the impact it had on her and the space it takes up in her memory, the episode with Wentworth seems much larger to Anne than the calendar says it was.
How eloquent could Anne Elliot have been! how eloquent, at least, were her wishes on the side of early warm attachment, and a cheerful confidence in futurity, against that over-anxious caution which seems to insult exertion and distrust Providence! She had been forced into prudence in her youth, she learned romance as she grew older: the natural sequel of an unnatural beginning. (4.9)
Anne's experience suggests that in some cases it's better for young people to make their own decisions – they might be mistaken, but older people don't necessarily know any better. While Anne has gotten wiser as she got older, that wisdom tells her to do something totally different than what the supposedly older-and-wiser people told her to do at the time.
Soon, however, she began to reason with herself, and try to be feeling less. Eight years, almost eight years had passed, since all had been given up. How absurd to be resuming the agitation which such an interval had banished into distance and indistinctness! What might not eight years do? Events of every description, changes, alienations, removals--all, all must be comprised in it, and oblivion of the past--how natural, how certain too! It included nearly a third part of her own life. Alas! with all her reasoning, she found, that to retentive feelings eight years may be little more than nothing. (7.28)
Emotion trumps reason here – despite her best efforts, Anne is unable to explain away her feelings. Just saying "this is the way things should be" doesn't actually make them so. Why does she think that Wentworth's feelings have been more reasonable than her own?
Whether former feelings were to be renewed must be brought to the proof; former times must undoubtedly be brought to the recollection of each; they could not but be reverted to; the year of their engagement could not but be named by him, in the little narratives or descriptions which conversation called forth. His profession qualified him, his disposition lead him, to talk; and "That was in the year six;" "That happened before I went to sea in the year six," occurred in the course of the first evening they spent together: and though his voice did not falter, and though she had no reason to suppose his eye wandering towards her while he spoke, Anne felt the utter impossibility, from her knowledge of his mind, that he could be unvisited by remembrance any more than herself. There must be the same immediate association of thought, though she was very far from conceiving it to be of equal pain. (8.2)
That reference to "equal pain" sneakily slipped in at the end suggests that Anne doesn't always read Wentworth's feelings correctly – his emotions are stronger than he lets on, even to Anne's close observation.
Yes; he had done it. She was in the carriage, and felt that he had placed her there, that his will and his hands had done it, that she owed it to his perception of her fatigue, and his resolution to give her rest. She was very much affected by the view of his disposition towards her, which all these things made apparent. This little circumstance seemed the completion of all that had gone before. She understood him. He could not forgive her, but he could not be unfeeling. Though condemning her for the past, and considering it with high and unjust resentment, though perfectly careless of her, and though becoming attached to another, still he could not see her suffer, without the desire of giving her relief. It was a remainder of former sentiment; it was an impulse of pure, though unacknowledged friendship; it was a proof of his own warm and amiable heart, which she could not contemplate without emotions so compounded of pleasure and pain, that she knew not which prevailed. (10.42)
After the previous passage where Anne doesn't see that Wentworth also has painful memories, her interpretation of his feelings here can't be entirely trusted – it's only her opinion, after all. Why is Anne so sure that she's reading Wentworth correctly? Why does she narrate his feelings to herself in such detail?
Though they had now been acquainted a month, she could not be satisfied that she really knew his character. [...] He certainly knew what was right, nor could she fix on any one article of moral duty evidently transgressed; but yet she would have been afraid to answer for his conduct. She distrusted the past, if not the present. [...] there had been a period of his life (and probably not a short one) when he had been, at least, careless in all serious matters; and, though he might now think very differently, who could answer for the true sentiments of a clever, cautious man, grown old enough to appreciate a fair character? How could it ever be ascertained that his mind was truly cleansed? (17.26)
Anne's concern here is less that Mr. Elliot is going to revert to his old bad ways, but that his past reveals something essential and unchangeable about his character – and even more, that she can never be sure that what he currently appears to be is what he'll still be in ten or twenty years.
"The last hours were certainly very painful," replied Anne; "but when pain is over, the remembrance of it often becomes a pleasure. One does not love a place the less for having suffered in it, unless it has been all suffering, nothing but suffering, which was by no means the case at Lyme. We were only in anxiety and distress during the last two hours, and previously there had been a great deal of enjoyment. So much novelty and beauty! I have travelled so little, that every fresh place would be interesting to me; but there is real beauty at Lyme; and in short" (with a faint blush at some recollections), "altogether my impressions of the place are very agreeable." (20.16)
Although Anne here says that memory can transform pain into pleasure, it seems for her the reverse has been more often true, as remembering the pleasures of her happy days with Wentworth became painful when she thought they were lost, never to be repeated.
The truth was, that Elizabeth had been long enough in Bath to understand the importance of a man of such an air and appearance as his. The past was nothing. The present was that Captain Wentworth would move about well in her drawing-room. The card was pointedly given, and Sir Walter and Elizabeth arose and disappeared. (22.63)
Even Elizabeth has to bow to merit, at least when public opinion says she should. Still, it's only Wentworth's "air and appearance" that matter to her: how he will look, and how those looks will reflect upon her as hostess, rather than who he is.
Soon words enough had passed between them to decide their direction towards the comparatively quiet and retired gravel walk, where the power of conversation would make the present hour a blessing indeed, and prepare it for all the immortality which the happiest recollections of their own future lives could bestow. There they exchanged again those feelings and those promises which had once before seemed to secure everything, but which had been followed by so many, many years of division and estrangement. There they returned again into the past, more exquisitely happy, perhaps, in their re-union, than when it had been first projected; more tender, more tried, more fixed in a knowledge of each other's character, truth, and attachment; more equal to act, more justified in acting. And there, as they slowly paced the gradual ascent, heedless of every group around them, seeing neither sauntering politicians, bustling housekeepers, flirting girls, nor nursery-maids and children, they could indulge in those retrospections and acknowledgements, and especially in those explanations of what had directly preceded the present moment, which were so poignant and so ceaseless in interest. All the little variations of the last week were gone through; and of yesterday and today there could scarcely be an end. (23.58)
There's a strange double-ness in this scene – not only do Anne and Wentworth talk about their memories of how they got to this point, they're also very conscious of how they will remember this present instant in the future. It's like they've lived on their memories for so long they can't help but think about how things will look from that future perspective.
"Perhaps I ought to have reasoned thus," he replied, "but I could not. I could not derive benefit from the late knowledge I had acquired of your character. I could not bring it into play; it was overwhelmed, buried, lost in those earlier feelings which I had been smarting under year after year. I could think of you only as one who had yielded, who had given me up, who had been influenced by any one rather than by me. I saw you with the very person who had guided you in that year of misery. I had no reason to believe her of less authority now. The force of habit was to be added." (23.72)
For Wentworth to make his peace with Anne, he has to give up the Anne in his memory in order to actually see Anne as she is in the present. The passage of time has cemented the past so much for him that the present's contradictions of it can barely make a dent.