Thank you for the request!
Because a belief system is a strong part of someone's outlook on life, to believe something for a period of time is to see things in life from that perspective. It is to understand how things work from that perspective and to develop thoughts and beliefs based on that mindset. Therefore, it is not easy emotionally to have that mindset suddenly change after so many years.
Changing one's mindset based on wrong beliefs is a loss. And since it is a loss, there is grief and mourning involved. Any loss, no matter if it is a person, thing or ideal, needs mourned. Letting yourself feel that loss is important.
Also, the loss of a mindset can cause a shift in your self confidence. You believed something for so long then you have to change what you think and feel. If you find out what you thought was wrong, then you may question yourself and your ability to know right from wrong or see through a deception.
Start addressing this by seeing that anyone can accept a certain mindset and feel it is the right one. Everyone has certain beliefs and ideals. That is part of being human. It is our ability to have an open mind that makes a difference.
Accepting that you are willing to change your mindset is also a good focus. Many people are not willing to change even when presented with evidence to the contrary. It shows flexibility and openness to be willing to see another side of things. This means that you are able to deal with your life in a more positive way and are able to "go with the flow" which makes for a more emotionally healthy outlook.