My interpretation:
Sally had already had one bad marriage and was in a dangerous relationship that she didn't know how to get out of. He was controlling and violent underneath his 'devotion', and objectified her. In a few years she would have been a wreck.
Carl was also at a very low ebb, MLC, bully of a boss, relationship with wife and kids in the doldrums.
Being with Carl, who is basically a kind man, gives Sally what she needs to get out of her bad relationship, and ultimately the strength to end both relationships properly.
Because Carl and his wife basically do love each other they survive. He gets a better job where he isn't bullied any more, and he appreciates his wife & kids more.
Carl is happy at the end because he can see she is stronger and happy, and this brings him some closure.
It was painful to watch, but I liked it.