The relationship between Yunior and Lola in this story is shown to be a relationship between two people that genuinely love each other. However, the problem is that Yunior, as a stereotypical Dominican male, is unable to remain faithful to her, and continues to have relationships and one night stands throughout their time together. This eventually comes to a climax after Oscar and his mother's death, when Yunior tried to do everything he could to make things work, but clearly was not able to restrain himself:
I wish I could say it worked out, that Oscar's death brought us together. I was just too much the mess, and after half a year of taking care of her mother Lola had what a lot of females call their Saturn Return. One day she called, asked me where I'd been the night before, and when I didn't have a god excuse, said, Goodbye, Yunior, please take good care of yourself...
There relationship is ruined by Yunior's repeated infidelity, and his inability to accept that. Note the way that Yunior does not explicitly refer to his infidelity in the quote above, only that he didn't have "a good excuse." This suggests that even now he is unwilling to acknowledge his own role in destroying their relationship.
As for Oscar and Ybon, this is a completely different relationship, as Oscar is completely entranced by Ybon and willing to do anything, even getting himself killed to pursue it. Ybon is a character who seems to find it hard to accept Oscar's love, and she does what she can to put him off, to be fair, but at the same time she is attracted to the kind of love and adoration that he offers. Even though she is a prostitute, Oscar is smitten, and he shows the kind of faithfulness to Ybon that Yunior is not able to offer Lola.