What is the Fight or Flight Response?
The fight or flight response is a normal physiological reaction to perceived danger. It is part of your survival instinct and in situations of danger it would mean that your body receives a boost of adrenaline which is helpful if you need to act fast or run.
Do you have an Overreactive Fight or Flight Response?
The problem with the fight or flight response is that it can become over-responsive and flare up when there is no real threat or danger. This is an over-reaction by your sympathetic nervous system. You can through practice get it down to normal reactions.

How do I get out of fight or flight mode?
The parasympathetic nervous system needs to kick in in order to calm you. This can be stimulated by things like meditation, mindfulness, breathing exercises, yoga, going for a walk. Start regular practice of one or more of these activities and you may find a noticeable change happens.
More Information
Further Help
If you need help and nothing is working for you go and talk to your doctor as they will be able to refer you for some CBT or may even prescribe something for you.