Combine this with the fact that our brains are wired to help us survive. A lot of the information gets interpreted as threatening and the brain processes it accordingly. As such the mechanisms of the brain tend to dismiss positive experiences unless they are particularly unique or interesting or intense. Instead the brains stores a ton of negative information with the aim of being able to survive the stress and wear and tear of life. Our lower brain centers complicate the matter further. The amygdala for one, will ring alarms bells if we tray and change behaviors that in the past have been less than desirable. It part of the reason we often experience anxiety when attempting change.
Though we are both wired and trained to think negatively about ourselves, it is not impossible to change. The main requirement for change is a willingness and determination to change. Then we try and reprogram ourselves via the unconscious to be more positive in outlook. Start by acknowledging that there is an unconscious and enter a relationship with it. Get to know it well, see how it functions then see if there isn’t a way to allow it to work for your benefit.