It is not easy to challenge core beliefs. You are right, they are developed over time, and are our automatic, unconscious response, much like an emotional reflex.
But they can be identified with effort and determination, and challenged.
I think for me mood plays a major role in whether they gain traction again or not.
We will probably carry them with us through life, but coupled with a dedication to balance insight and allowing yourself permission to let them revisit conditionally, it is possible to gain strength from the very things that once limited us.
The reflex perhaps is automatic, but with the work a secondary thought process can assist in limiting how much they effect self concept.
Not easy, not something I have mastered. But identifying them and understanding their development does help.