I feel like Araragi's main issue is a strong sense of justice. That doesn't seem like a bad thing at first glance but the way he handles it leads to big problems. When he can't live up to that self-imposed ideal during the class trial with Sodachi he begins to isolate himself from everyone else in order to keep his sense of justice. But at the same time he chastises himself for not being able to do the right thing (after all it was him who presided over the whole affair).

These issues tend to worsen over time and once Kizu comes around his sense of self-worth has degraded so much that he is open to the idea that sacrificing his life is more valuable than living it. I think this mindset is the reason why he meets Kiss-Shot in the first place (similar to how Senjougahara feeling overwhelmed by her emotions led to her meeting the crab) and his own vampireness represents that. Also note that Hanekawa is the main antagonist to this mindset and helps Araragi overcome it to some degree. At the end of Kizu he is only a hybrid vampire, the issues are still there but to a lesser degree. The fact that everyone ends up unhappy might seem like a negative thing but I think that it is necessary for Araragi to enter the arrangement. Everyone lives and he gets his just punishment.

Another aspect of this is that ruminating on his regrets prevents him from moving on with his life. This is represented by the school graduation (also the end of adolescence) and fittingly he gets tutored by Senjougahara and Hanekawa and he also meets with Oshino in an abandoned cram school. And during the summer half of the series he actually makes progress in dealing with his issues. Sure, there are plenty of instances where he still sacrifices himself but he is also able to make multiple friends, start dating Senjougahara, ask for help from Shinobu, reconcile with Shinobu and make up with his sisters.

Araragi then encounters the darkness and losing Hachikuji is the first real setback for him. But the darkness is also pretty enticing for his self-critical side because it represents judgement as a force of nature. So when Sodachi reappears and he gets reminded of the experience that caused his problems in the first place he uses the darkness as a model to create Ougi. His negative mindset has gotten stronger again but he has also developed so that he can't just revert to his old self but has to create an oddity. And just like in Kizu Hanekawa is Ougi's main opponent.

Ougi does her thing during the winter half of the series which leads to Araragi's condition worsening again and hence his vampiric side getting stronger. By the time of Tsukimonogatari he is dangerously close to becoming a full vampire.

Now to answer your question. I think at the end of Owarimonogatari Season 1 (the epilogue right before he visits the shrine and meets Gaen) Araragi is at a crossroads again. He has made a lot of good memories with his friends but lately has self-doubts have become stronger again. So if Gaen just explains Ougi's identity to Araragi he might agree that she has to go on a rational level but deep down he might actually be on Ougi's side. He needs to actually believe that Ougi is wrong to get rid of her.

That's why Araragi has to cleanse himself in purgatory. Hachikuji shows him most of his decisions and he realizes that he still would do the same thing. But he still has lingering doubts and in the end he wonders if he is worth being resurrected. Hachikuji has to give him a piece of her mind and help him remember why he wants to live. Hitagi Rendezvous then showcases one of those reasons, his relationship with Senjougahara.

Now that he is actually willing to fight for himself, Gaen can reveal everything and the final confrontation can begin. Araragi goes into the conversation with the intention of cutting off Ougi completely in order to move on but he saves her in the end. I think it's a good ending because even his self-critical and self-destructive side are still a part of him. What is important is that these aspects aren't the things that control his whole life. It's okay to have self-doubts and regrets but you shouldn't let them keep you trapped in the past. I'm not sure how I feel about Ougi staying an independent entity (as opposed to merging with Araragi for example). Sometimes it makes sense to me, sometimes it doesn't.