You can write your negative self-beliefs and then ask yourself:
• Who made me believe this?
• What experience made me believe this?
You can close your eyes, focus on the belief, and ask the questions until you get the answers.
Instead of singing a mantra, like ”Om", repeat "who made me believe this?”, several times, and until your subconscious answers the question with images and mind movies related to the past.
Once you know it and re-experience it as an outsider, an observer, it won't have more power over you.