In most cases, the sentence would likely be, "I can sabotage myself," but in the context of the song, the sentence might be:
I can sabotage myself by myself.
"I can sabotage myself" means that the speaker is capable of self-sabotage. The addition of "by myself" is redundant, but emphasizes that the speaker does not need any help in the activity.
Less redundant ways to say the same thing would be:
I can sabotage myself on my own.
I can sabotage myself without any help.
etc.
Your other sentences might be
She's so crazy that she writes letters to give to the mailman that will be delivered back to her.
She looks at the mirror and breaks it after seeing herself looking back at her.
The first sentence should not have "herself", because the action of delivering the letters now originates elsewhere.
The second sentence sounds more idiomatic with "looking back at her", because it portrays the reflection as another entity.