Can not anger be let go of? It's in the past already, can not change what happend.
I think this is a "pie in the sky" view of things. For certain situations, this attitude or approach may make sense. For others, it does not.
Take, for example, the instance in which a friend asks to borrow some cash. They promise to pay you back, and ask for like $500 for an "emergency." Well, some time passes and you ask about it a few times, and they keep postponing, and then later they come back to you asking for another $250 for something else. Now... you could just "forgive" them the fact that they can't seem to pay you back for the $500, and treat the $250 as a brand new instance of them asking you for a loan on good faith. But that would be incredibly naive. A better idea might be to put them to the test, learn from what was potentially your prior mistake and tell them you will loan then the $250 as soon as they repay the $500. Remind them that there is an outstanding monetary relationship between you still on the table, so that you don't get further taken advantage of (if that is what it feels like is happening). According to your recommendations, we might just say "but that's in the past" and go ahead and keep loaning them more and more if we are willing and able. But that is unsustainable in the long term... and the person who keeps asking for the money is potentially learning that bad habits pay off.