2 Corinthians 7:10
Regret is a bear. Every time someone stumbles, they go through a moment of reckoning in which they realize the mistake they made. And it hurts.
There are two ways to deal with this. The first is to spend a while in self pity and hate yourself, but then to get on with your life.
The second way is to repent. This means that you realize your mistake, turn it over to Christ, accept his grace, and let him help you change. Some argue that to not spend time dwelling on the fact that you sinned belittles Christ's death on the cross. CS Lewis argued that to do otherwise is the true belittlement of the Sacrifice. After all, "It is finished."