The Big "I'm Sorry!"

I don't know where to begin as far as forgiveness. I had heard it time after time those all famous and familiar words "I'm sorry". Why do those words seem to be so easy coming out of his mouth, especially when he knows I am tired of hearing it. If he were truly sorry, then he wouldn't have kept cheating on me over and over again. After the first time and the first "I'm sorry" he should have been sorry enough to try and straighten out this mess he helped to put us in. Where do you start forgiving? How can you just pick up the big mess that he's piled in front of you and just act like things are ok? What can I do to even start the process of forgiving when I 'm hurting so badly?