Ending (Spoilers)

I think you're missing a big part, which is that Knightley's character did everything she did "knowing" that Worthington's character wouldn't cheat on her.

So she went out with her Parisian fling, kissed him, lied about her husband, and slept with him (everything but sex basically) all while thinking her husband was just sleeping in bed or working.

Also, look twice at the note she writes to him. "I might have overreacted". Even then she can't admit when she's wrong. Is she writing the note for him, or for herself?

You seemed to have missed a lot.

The marriage was in a "dark valley" to begin with because Michael was Joanna's "second choice" and basically a "nice, safe" rebound relationship for her while she secretly pined for Alex - the guy who ignored her in Paris (other than f'in her so well) to the point she left him to make the point - which dear old Alex never got - since HE went right on screwing other women. Such a "hunk". A very shallow man, actually.)

Michael getting drunk and his one night stand isn't the problem - Joanna being obsessed with her past lover is the problem - right from the very beginning - and for years.

All the clues are there now for Michael to discover Joanna's secret love(r) - not the other way. At a minimum they need a separation and some "space" to decide what they really want to do. Personally, I wouldn't stay married to her. She is just too stupid.

And she is WAY too joyless and a total downer to be around. Plus she smokes. She "deserves" Alex totally. Six months with him and they would be at each other's throats - and he definitely would be back to his "casual sex with others" ways.

This movie was ended perfectly.

I think you're all missing something. Jo tells Alex in the kitchen that there has been many times she has "almost told Michael" about him. That little gasp of breath at the end of the movie I believe is to make the viewer question whether she is about to finally tell him.