It's "I wish I knew how to quit you!"

Not:

• I can't quit you!
• I don't know how to quit you!
• I wish I could quit you!
• Why can't I quit you?
• I'd kind of like to quit you, but I can't.
• I want to quit you!
• Why won't you let me quit you?
• I'm not a quitter, but I want to quit you!
• etc. etc.

This makes me nuts. What's good about the line is the double impossibility in it, and the reason that's good is that it echoes the theme and mood of the whole damn film.

Respect Larry McMurtry, people. The exceptionally tortured dialogue in Brokeback is a serious achievement.