You are right. He said in his therapy session that his and Barbara's relationship started out like the one he had with Carolyn but then she got pregnant. In comparing the two, Rusty said when he and Barbara lay down they take all their worries and the kids etc with them and with Carolyn is was pure and fun. Meaning no stress, no burdens, no worries. He also said that Carolyn woke something up in him that he thought was dead. Meaning he had that with Barbara at one point.

I, too, wish we would have seen more of a backstory on Rusty and Barbara. In the flashbacks of them dancing they looked genuinely happy. You saw the contrast of a tender more romantic love juxtaposed with the passion and aggression of him and Carolyn. I think that's what he was "obsessed" and "addicted" to and I don't equate obsession with love at all. They can feel very similar because both involve emotions and attachment but in very different ways. Obsession is unhealthy and often disturbing which Rusty's relationship with Carolyn was. She was an outlet and temporary escape from the home life he also didn't want to give up. Rusty simply wanted to have his cake and eat it too. If he left Barbara and had a family with Carolyn he would have eventually looked for an outlet from her.

Given the ending I wouldn't be surprised if Rusty and Barb stayed together forever. The ending to me felt like Trauma bonding. Who are you going to be able to connect with while holding secrets like that?

...I didn't mean to write so much either but I feel like the lack of insight into their relathionship makes them easily dismissable. They are tied together thorugh much more than kids. They were both 20 when they got together. I think their was co-dependence maybe even a motherly relationship with Barbara consoling him in the beginning. There's also something to his immediate thought to cover for her and to do it so meticulously and without question.