I wanted to know if it was possible to use self-hypnosis on myself to forget my parents existence

No, any level of forgetting will only be temp at best

Why would you want to forget about them? Self hypnosis is usually used as a way of relaxing but not forgetting. It is about changing attitudes, mainly your own. Its about growth and changing your bad habits.

Kitten, talk to us. Is there a reason why you want to forget about your parents existence? How old are you? Have you spoken to a teacher about this? Or anyone at all?