The brain's primary function is to keep you alive. When facing changes, even though you are in pain, the brain has a lot of evidences that you will survive (despite a lot of pain), but when you change, it does not know if you can make it --> So the first reaction when changing is the intense increase in fear and all pains/emotions
If you have never held a boundary before, the holding of a new boundary will initially feel like terror. So what. To change, you must understand there is an other-worldly difference between being in danger and feeling in danger.