The title is a lyric from Caught in the Middle by Paramore.
I know lyrics can break grammatical rules, but I want to know the correct way of using reflexive/intensive pronoun. So, should it be
I can sabotage myself by me.
I can sabotage myself by myself.
I'd like to add too my own examples
She's so crazy that she writes letters to give to the mailman, which will be delivered back to her/self.
She looks at mirror and brakes it after seeing herself looking at her/self.
Unrelated additional question but please answer: What's the difference in meaning if I don't put an article (definite or indefinite) between "at" and "mirror"? Is mirror a countable noun?