Experiences you have through life can have a big affect on you - they often shape the way you think and see the world. Sometimes they reshape the way you think about yourself.
From when you are a child and as you grow through your teens, you often have little control over what you experience. Some experiences can be bad and even damaging.
Most of the time you learn coping skills from these bad experiences but sometimes they have such an effect that you don't learn how to cope at all.
When this happens you become more and more vulnerable to stress. Then the smallest amount of stress can make life very difficult. You have less ability to deal with your own feelings and sometimes they create serious confusion. You become very vulnerable to stress.