Our core beliefs of self, others, and the future are our filters to reality. Our beliefs influence the brain to focus only on the events that match such beliefs and ignore the rest. This makes the beliefs deeper and more believable with time. When you have a belief that you are not good enough, unlovable, or powerless your brain would actively seek information that supports such thoughts and overlook and not store the information that does not align.
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Mental Health is an invisible disease that leaves signature in the brain. PET scan for brain activity for people with different mental health disorders: Mayo Clinic, NIMH.