In the book Gates of Repentance [by Rabbainu Yona of Grondi] is brought the idea that repentance fixes things in the past. That is to say you might have found yourself in a situation that you realize is a result of past mistakes. So according to Rabainu Yona, if one regrets the past mistakes and accepts on himself now to do better, that reaches back into the past. It makes those mistakes as if they never were. And thus the results of those mistakes disappears.
So it always is worthwhile to think about what one's mistakes were. This is not a way of getting downtrodden , but rather a way of getting rid of the effects of one's mistakes.
