Figure 1 shows the path of mental proliferation. The numbers from 1-4 indicates the direction of the flow of this process. Also thoughts can be retrieved from the memory to add to this process.
Figure 2 shows where we can fix this problem of mental proliferation. It is mainly The Foundations of Mindfulness and insight meditation (yellow and green bubbles), taught by the Buddha about 2600 ago. Parts of mindfulness meditation is now used in many clinical applications as a secular form and it is now called Mindfulness Based Stress Reduction (MBSR) and Mindfulness Based Cognitive Therapy (MBCT).
A cancer is generally defined as "uninhibited proliferation of cells". As we all know this can cause all sorts of problems in the body. Similarly in the mind too uninhibited mental proliferation of thoughts can cause all sorts of problems in the mind. Thoughts about the past such as "I should not have done this or that..." may lead someone to depression. In the same way uncontrollable proliferation of thoughts about the future such "what if this happens..." may take someone to anxiety states. This the cause of our unhappy states of mind.
So can we understand how this "mental proliferation" work in the mind?
In Madhupindika Sutta: The Ball of Honey Buddha gives us great insights about this process of "mental proliferation" or term called "papañca" in Pali.
Thanissaro Bhikkhu says "What one feels, one perceives (labels in the mind). What one perceives, one thinks about. What one thinks about, one "papañcizes." Through the process of papañca, the agent then becomes a victim of his/her own patterns of thinking: Based on what a person papañcizes, the perceptions & categories of papañca assail him/her with regard to past, present, & future forms cognizable via the eye [as with the remaining senses]
contact > feeling > perception > thinking (mental formations) > the perceptions & categories of papañca (past, present and the future).