It is so natural and so easy to get bogged down in the idea that what we've done or what we've been through is simply too much for grace to overcome. We declare ourselves untouchable, unreachable, or un-healable. Remember Paul's words: "Forget what is behind."
Refusing to forget these things, in the end, this is just a subtle form of pride. In doing this we assume that we're the one person who is too much of a problem for Jesus. We're the one nut He can't crack. We've got the one situation for which the cross of Christ is inadequate.
Oh sure, He can save Paul, He can deliver Peter, and He can make all things new. But not me! I've got grace's kryptonite. And this is how refusing to forget what lies behind is prideful.
On the flip side of that, because of the cross and because salvation is from Christ alone, we can actually come to boast all the more in our dark pasts. No, we do not boast as a way of glorifying sin or championing ourselves, but instead to magnify the wonders of the grace and mercy of Christ.~ Matt Chandler in To Live is Christ