I am reading a book on this matter (The Diamond Cutter - Geshe Michael Roach, essentially a very modernized commentary on the Diamond Cutter Sutra, also known as the diamond sutra).

I have been starting to understand how our actions shape our Perception. (Example: One Person perceiving a raging boss positively as a strong leader, another perceiving him negatively as an oppressive asshole, both respectively due to their past actions)

An Example.

Cause: Being rough in the way you treat other, not caring how they will feel about what you say or how you say it causes, both obvious to their face, or hidden behind false praise or sarcasm.

Result: Your Business Competitors are merciless and you always seem to lag behind in most every situation.

This makes sense when reading it, but how can myself I understand what causes what? Especially in the current moment?

Also, it was mentioned that once you create an action, it will always yield the corresponding result. Right now, most of us here are hopefully trying to stop creating these negative causes, but what about the ones from the past that still await us? How do we escape/negate or reduce their effects? How do we recognize in what way our actions will shape our future perception of this world? How do we recognize what the cause for our current suffering is, in order to stop creating more of it and trying to create Karma for the opposite?

Also, In what way do vajrayana prostations/other ngondro purify past karma? How is that possible?