Lately I have been thinking about the monsters that frightened myself as a child.
I’m supposed to change “myself” to “me” because using “myself” after “frighten” is not standard grammar. But, I still don’t understand why. Why is"myself" not grammartically correct here?
To use the pronoun ‘myself’ in your sentence, the subject and the object would have to be the same. You would need a reflexive verb. Since the subject is ‘the monsters’, they have frightened you (not yourself).
Of course, you can frighten yourself (it can happen 
!), yet it’s not the case in your example – when I was a little girl, I used to try to frighten myself by pulling the most horrible faces in the mirror (now there’s no need for that any more :lol: )!