Yeah, try to remember to remind yourself that the past is gone, and it can be gone as if it never existed, if you choose. Morally-framed people as you say can be Ps in the A because of their hypocrisy and self-righteousness. But the intent behind the phrase is sound--don't let the past bother you (or try not to--I know it can be hard), and if it rears its ugly head, just remember who you are today and that who you are today isn't who you were then.

In other words, learn from the past, but don't worry about the past. Don't let it be a shackle to keep you back from your full potential. One day at a time, keep reminding yourself of this until it sticks.

Nothing is impossible, only improbable, and if you change the variables, you can change the probabilities. Just be gentle with yourself.

And above all else...