Thinking of making a change? "Slip out the back, Jack" just not literary enough for you? The Atlantic has some examples of break-up letters by literary figures from history. For example, Anaïs Nin:
But in the middle of this fiery and marvellous give and take, going out with you was like going out with a priest. The contrast in temperature was too great. So I waited for my first chance to break—not wanting to leave you alone.
You ought to know my value better than to think I can be jealous of the poor American woman who has lost her man to me continually since I am here
Or Simone de Beauvoir to Nelson Algren:
I want to tell you only two things before leaving, and then I’ll not speak about it any more, I promise. First, I hope so much, I want and need so much to see you again, some day. But, remember, please, I shall never more ask to see you—not from any pride since I have none with you, as you know, but our meeting will mean something only when you wish it. So, I’ll wait. When you’ll wish it, just tell. I shall not assume that you love me anew, not even that you have to sleep with me, and we have not to stay together such a long time—just as you feel, and when you feel.
Right now, I'm thinking of what to say to the 2012 Chicago Bears, who just couldn't give us what we needed this year.