Which Film is this?

So a movie plot arrived in my head tonight. I'm sure it's at least one movie. Probably many. But I can't think of any. Except maybe the trailer for Funny People (but I haven't seen the film so I don't know if this is what happens). So if you guys know a film like this let me know what it is:

Boy meets Girl.
Boy likes Girl.
Boy and Girl are friends, and do or do not get together.
Boy and Girl break up/never got together, but Boy always loves Girl.
Boy leaves Girl's life and/or Girl meets Guy who is horrible to her, but she falls in love with him.
Girl marries Idiot Man
After a few years Girl is unhappy with Idiot Man.
Boy comes back into Girl's life.
Boy and Girl have an affair.

And then obviously they are happy till Idiot Man finds out and things go pear shaped.

So what's the film? I know there are plenty of Girl is married to Idiot, Girl has affair. But I can't think of Girl marries Idiot has affair with old nice Boy who loved her.

Any thoughts?