We only get a limited amount of energy to function every day. Or rather, our brain gets that energy to keep taming our body throughout the day.
What happens when we involve ourselves too much in negative thoughts?
We use a significant amount of that energy into those thoughts.
These thoughts can be anything from something that has happened to you in the past to something that you're worried about in the future.
You're then left with a depreciated amount of energy to do the useful thinking.
This is called an energy leak.
The more and more you stress out or spend time dwelling in the past, the more energy you leak.
Like all leaks, you need to fix this one before things get out of control.
Stop worrying too much and start spending your daily energy to useful thinking that'll help you progress in life.