Let’s say we have a glass with an inch of sand at the bottom of the glass. When we pick up the glass and swirl it around, the sand flows freely around all of the water, almost mixing equally in the entire glass. When the glass is set down, after a while the sand all settles to the bottom. You can then see clearly through the glass. This how the soot at the bottom of your heart is. When there is no stress, or things are going well, the glass is undisturbed. As soon as something happens it is like the glass is being picked up and shaken, and all the soot or sand is again flowing throughout the glass or the mind.

The sand in the bottom are all the unsettled conflict that is in your past. When something touches in that part of your mind, it is tossed into the mind it swirls around in your consciousness and creates all those negative emotions

One idea here is the last straw