How can you forgive the unforgivable?

How did you forgive people who have done unforgivable things to you? Where do you draw the line between a forgivable mistake and a breaking up mistake? The anger and hurt can be very confusing. How do you ask for forgiveness after you've done the unforgivable? How easy or difficult is it for you to forgive someone?
How difficult is it for you to let something go? Should we always forgive people who hurt us? What do people do that's unforgivable?