I felt the story could have been better. The actors in the movie have done their job really well. It showcases the actual toxicity of many long term relationships. I felt that Mel and Dan were very dependent on each other and that made the relationship kind of toxic.
Sometimes we get too comfortable if we have been involved with somebody for a long time because it makes us feel secure and protected and that's what the characters in the movie were feeling too.They both knew deep inside that the relationship has lost its spark but despite all that, they continue being together because breaking up or separating from somebody you are dependent for for every little thing is painful.