If someone said you look ugly with your purple hair. You most likely wouldn't care because you don't have purple hair. If they said you suck, you'd believe it because you don't think you're wholly good. Perhaps, you suck to a degree. If they asked whether you think your parents suck, you may argue in disagreement. But if they asked whether your parents are wholly good, you'd still disagree because they have flaws you may be aware of.
It's fascinating how we tend to judge ourselves harshly yet easily extend grace to anyone but ourselves. With this bias, we rob ourselves of the very mercies we need to live a life free from internalizing beliefs others have about us.
Briefly put, our capacity for grace is often directed outward, while our capacity for judgment is directed inward. We often tend to instinctively grant others the grace of being imperfect without being worthless, yet we often refuse to grant ourselves that same grace by allowing external opinions to define our self-worth.