Unmanaged stress has specific effects on the brain. It can cause neurons in your hippocampus to die.  You may recall that your hippocampus is the area deep within the brain that is critical in regulating memory and emotions.  When those functions are damaged by chronic or long-term stress, your cognitive function (ability to reason and think) declines, damaging your ability to remember and learn.  People who experience prolonged stress experience difficulties with both short- and long-term memory.  Compared to those with more manageable levels of stress, they have difficulty doing math, processing language, and connecting new information to existing knowledge.   Prolonged stress hurts cognitive learning, declarative memory, and problem-solving. 

Beyond how you learn in the moment, chronic stress can make it difficult to adapt and make connections between new and old information and situations.  Over time, prolonged stress hinders virtually every aspect of learning.

If you and a friend are both trying to maintain a certain grade in a class, the challenge to keep up provides a little stress, just enough to motivate you to do your best.  At the same time, if you and a classmate battle to prove who is best, and failure to win the battle will cause humiliation and ridicule, the stress of the ongoing battle may undermine your ability to learn.  You will lose more than the competition with your friend. Chronic feelings of being overwhelmed, behind, confused, or out of control can create a pattern of stress that compromises your ability to learn.