What is a cognitive dissonance ?
A cognitive dissonance is a kind of mental stress or discomfort due to a conflictual new reality perception, beliefs or understanding that is confronted with the considered actual reality perception, beliefs or understanding.
In other terms it occurs when our thoughts need to change because a new informations that is considered as true want to replace the old informations that is also considered as true because (the logic would consider the old information as false) but because of some feeling (often negative) or a too important reconsideration of facts and thoughts, both perceptions side are accepted but the fact that there can only be one real side is accepted too. So both side are kept.
The sentence is a bit tricky but for resuming it's a bit like being at the right side and go on the left side but also stay in the right side, yes it's not possible so it make a dissonance and in some case split "personality" to stay in both right and left side.
This is a kind of psychological trauma.
What do to when a cognitive dissonance occurs ?
You can breath, take your side and be confident, a lot of surprising thing are, a shift in perception can take some times to occurs.
It's better to take some time in replacing our belief system that in refusing the new one on ignoring or finding false reason to continue the believing the considered "wrong way".
Often emotions are more powerful than fact to induce a perception shift.
(The image is just for kidding, source here, author is @mearone)