Poseidon's summer with Sally Jackson taught him much about who he was and what he wanted to be. He was so torn up about not being able to see her again that he very quickly went on the rebound, resulting in Percy's half-brother Tyson. After he managed to recover however, the god of the sea gave himself time to think on his immortal life, and how he really wanted to spend it. He had a goddess wife and an immortal son, both of whom he hadn't spent a long period of time with in ages. Guilt would wash over him whenever he thought about it.
The first thing Poseidon decided was that he was going to take his oath to not have any more kids seriously, hence why after Tyson he hadn't had anymore kids. He started spending more and more time with his wife and immortal son, quickly making up for lost time and confessing the love he had felt for another to his immortal wife. Poseidon could tell it hurt her, just as it did whenever he had cheated on her in the past. But things got better when he started to prove he could be a better man, and eventually Poseidon, Amphitrite and their son Triton were closer than ever.
When Percy grew up and joined camp half-blood, Poseidon watched over him and soon after Tyson and they learned to fight for themselves and both helped save the world. Percy soon ended up meeting Amphitrite and while she was cold during their first meeting, not that he could blame her, she quickly warmed up to his son and he could tell she now loved the eighteen-year-old like he was her own. It was the same with Tyson, though she didn't meet him until later on.
He learned throughout the years that Sally ended up using Medusa's head on that prick she married, Dave or something like that. He didn't tend to remember the names of abusers. She later remarried a handsome man named Paul, and Poseidon by that point couldn't have been more happy for her. He had loved the woman, he would even admit that he still did. But she deserved better than what he could provide, and he had a wife of his own whom he had realized he loved just as much as he loved Sally. The way Sally smiled at him told him she was in the same situation and felt the same way he did, which couldn't make him happier.
Overall, Poseidon was a happier and much better god than he had been in centuries. And it was all thanks to Sally Jackson.