Internal sources of Stress are caused by how we think internally about our lives and our behaviours and our perceptions.  For example, if someone believes that they are a failure, that they are not a good person, that life has not given them a fair chance, if they compare themselves with a sibling or friend or colleague and find themselves lacking that leads to Stress that is generated from internal sources, these being our thoughts and the way we reflect on our lives.  If we are self critical, that can lead into a circular spiral which is ongoing endlessly and therefore, exhausting.  This can create self generated stress which can become overwhelming.