As an ADHDer, I relate. I heard the phrase much better expressed as "if you don't set clear boundaries about how you expect to be treated it can make it easier for other people who won't respect your boundaries to treat you badly."

I don't think too much about if I "love myself" - like you say, that smacks of narcissism and we're all a little bit narcissistic. But, I do sometimes think, "Hey, that was disrespectful - I won't allow that person to treat me like that again," and that's a healthy perspective.