The ending definitely complexes the film. He's "succeeded" but at what cost? His mental health, his family, social life. I know it's ultimately about the cycle of abuse, and that's why the movie is so unique. He never breaks the cycle of abuse, in fact it ends on the "honeymoon" phase of the cycle, i.e. the desperate hope that you've satisfied your abuser, that things have worked out and they like you and won't hurt you.

I agree with you. I understand how it might be easy to think that the film had a happy ending if you didn't pay enough attention during the film.