We are always starting down our pasts when we look to our futures. There is no past that vanishes once it becomes a memory—stories last forever until forgotten, and the stories we don’t forget will last forever. Swallow down your memory, reject a past. An end, a beginning.
All you can do is make the choice to move forward.
The story of Sodachi Okikura happens two months after the events of Shinobu Mail. In these two months, we see an Araragi far removed from his stance at the end of the story in which he defeated Kiss-Shot Acerola-orion Heart-under-blade’s first minion, an Araragi who claims that his union with Shinobu will bring nothing but unhappiness and misfortune to him and to those around him. Instead we are shown an Araragi who has seemingly embraced his own happiness, who seemingly is no longer accompanied by Shinobu (although we know their partnership still exists three months even further into the future during the events of Hitagi end). 
What brought Araragi to this place, a place where he can echo Ononoki’s philosophies on happiness to Sodachi as if they were his own? There are no stories in this time yet. Perhaps all that can be guessed at is that Araragi finally has come to understand that he is not a special person, merely one who is special for some. Not just Senjougahara, but for all the girls who have gathered themselves around him. (Of course, it is Senjougahara who has the most power to help him understand this.)
Shinobu Mail is an arc packed with callbacks and callforwards. Gaen levels the same harsh question at Araragi that Shinobu once did—will it be the golden-haired vampire, the Tsundere-chan, or the Cat? Ononoki gives Araragi the answer that will help him just a few months later as he faces his past from long ago. Time is twisting forwards and backwards, and the First One leads the gyrations of the clock.
Shinobu is exactly right when she tells Kanbaru that the First One doesn’t love her—but love is not the only feeling that binds people together. All of Monogatari is proof of that. Love, lust, friendship, family, a desire for companionship, servitude… the connections between humans are not so simple as romance. Shinobu may not be human, strictly speaking, but that doesn’t mean that these other forms of connection are any less potent for her.
The First One cannot thrive as a human any longer—he stands on the very sun that is his bane, proof of his inhumanity. Revenge or simply a need to be acknowledged; he is driven back to Shinobu, who would rather avoid the difficulty of making a choice. In contrast to Araragi, whose reaction to choices is always to seek the option that will allow him to take everything, Shinobu would rather not make a decision at all. The status quo and stasis of immortality, as it were. A life that goes on forever.
Shinobu, I think, can’t really be blamed for her cowardice and for her desire to perpetuate more of the same, but selfishness doesn’t work if you’re not alone—as Kanbaru points out—and Shinobu is not alone. Shinobu’s situation is the exact opposite of Sodachi’s. Because she has people, she can’t give herself over to hopelessness and misery. It will hurt too many people.
Until Hanekawa and Araragi show up in Sodachi’s life, that’s not an opportunity afforded to her. But Sodachi leaves at the end of her arc, passes beyond Araragi and Hanekawa’s ability to help her or be there for her. She strikes out on her own into a lonely world… but maybe she can find happiness out there on her own. Maybe she can find someone to stand alongside her. But the important thing is that she make the choice to try to be happy, to give up on the equation of misery to which she’s committed herself for so long.
In Sodachi’s departure, we perhaps see a vision of Shinobu’s future life. Shinobu has been with Araragi long enough to understand that she is not the only being in his life with whom he shares a special bond. Mayoi may be gone, but there’s still Hanekawa and Kanbaru and Senjougahara. Shinobu can’t guarantee herself that Araragi won’t leave her someday.
You could make the argument that Shinobu is only doing what is necessary to keep herself happy. If she likes her life now, if she likes her current partner and if she doesn’t want things to change, why should she face the First One? Why should she risk her own happiness to bring closure to that relationship? The answer, perhaps, lies in Gaen’s lecture to Araragi, “A huge flaw that makes you say, ‘You’re wrong to convict someone that thinks you’re special.'”
Gaen’s words are circular and overly complicated, but all she’s saying is that sometimes ties must be broken. A choice must be made.
Araragi must make a choice between Shinobu and his friend and his lover. Shinobu must make a choice between her past minion and her present master. Sodachi must make the choice to cling to the misery of her awful past or to try to find happiness in an unknown future. Araragi and Shinobu do this at the same time that Hanekawa is making her own choice. Senjougahara made her choice long ago. In some ways, so has Kanbaru.
Gaen knows everything. Hanekawa only knows what she knows. Ougi doesn’t know anything (it’s what Araragi knows). Knowledge brings empowerment, but only if you have the courage to face it. Kanbaru never tells Shinobu anything Shinobu doesn’t know. But Shinobu doesn’t have the courage to do anything more than preserve her current happiness. She needs validation before she can act.
The Hanekawa of the Sodachi arcs no longer needs validation. She knows what she knows, but she’s also willing to face what she knows—even if the truth is horrible. Having acknowledged herself—having made her choice—she acts with purpose and confidence. Hanekawa has been criticized (both by Ougi and by watchers) of needing to fallback on using her sex appeal to compel Araragi to go with her. It’s a patently ridiculous complaint. In Araragi’s own words, it’s not the temptation that helps him make his choice but the urgency that the option presents.
Hanekawa knows Araragi. She knows what she knows. She can act. When Sodachi is helped to find the courage to know what she knows, she likewise can act. Likewise for Shinobu.
In the end, it’s not just having knowledge that matters (think of Gaen’s long, long explanations or the drawn-out nature of Shinobu )—but having the courage to act on that knowledge. Having the courage to make a choice (whether that choice be a lie, a truth, or otherwise). Knowledge alone is not enough. But knowledge and courage together can bring…
 I’ve been informed that Araragi’s conversation with Ononoki at the very end of episode 12 actually occurs after Tsukimonogatari (and therefore, also after the Sodachi arcs). In this case, the positions are reversed, but the point remains the same. Ononoki throwing Araragi’s own words back at him—indeed, the lesson of this entire season!—just mean that Araragi needs to relearn a lesson he once knew. The fact that Araragi forgets this makes the lesson no less valid.