Tom Paxton's Leaving London, has the right sort of bitter sweet, it's all over feel.
However as said before Don't think Twice, does it for me too. It contains my all time favourite lines on this subject.

Well I ain't saying you treated me unkind,
You could've done better but I don't mind,
You just kinda wasted my precious time,
But don't think twice it's all right.

I think he might just have been feeling bitter about that old flame!!

Giok