The Limbic System deals with and regulates emotions, memory, and sexual arousal. When it is not working properly, you will have feelings of depression and lethargy. The frontal lobe allows us to think, decision-make, plan, and problem-solve. Another part of the brain, the basal ganglia, affects habit formation, reward system, and skill learning. When it is not functioning properly, it can cause one to experience anxiety and or depressive symptoms.
The Limbic Brain or System is of particular interest to autism, adhd, and anxiety because it is often an overactive Limbic brain or under active prefrontal cortex signalling that causes anxiety, panic and fight or flight.
The Limbic Brains is also known as the lizard brain, it lies between the brain stem and the neocortex, it is more primal than the relatively newer neo cortex (which is rational thinking), the problem is the brain operates in a hierarchy of priority. This means the limbic brain (emotions) will easily override neocortical functions, this is one of the big reasons why anxiety is not easily defeated with neurotypical pep-talk (psychologists).
The issue for people afflicted with severe anxiety related to autism, adhd, anxiety and even brain damage is that the ability to regulate the limbic system is compromised, this means the limbic system will over respond to things that are normally not scary or stimulating to normal functioning beings. This is a BRAIN CHEMISTRY PROBLEM and not a psycholoical lack of will or training.