I disagree partly with the original post. I think that when you start to dwell on something- BEFORE you let it go, it is important to first give yourself a positive thought on it, THEN let it go.
E.g. if you regretted that you screwed up a conversation... You find yourself thinking "they thought i was an idiot, I am terrible at this etc..."
First tell yourself STOP- THEN give yourself positive realistic thoughts - E.g. "It's okay- Everyone has moments like this. Also, I am an anxious person so it's natural that I reacted like that. I am improving anyway, and I did act well during another part of the conversation. They probably didn't even think much about it." THEN make yourself move on.
I think if you just repress everything without replacing the negative with positive, eventually you will have a crash where something bad happens that you can NOT just push aside, and suddenly you feel horrible about everything as everything that you pushed under the rug is coming back to you.
Anyway, basically I just wanted to add that in my opinion, there should be an extra step before "letting it go."