I've always thought there were at least two ways to interpret the ending:
1-Severine being thrust back to her housewife life, she must now care for her disabled husband, in essence being punished for her deeds. If she felt "trapped" before, she is positively smothered now.
2-With her husband being in her care, she feels liberated. She is in control of the marriage, she makes decisions and she no longer has to be the angelic posterwoman that she was rebelling against before. So, the pressure is off her to live up to any standards.
I myself fancy #2...but of course, the ending is open to numerous interpretations and I choose to hang a "happy" ending on it.