Self-Doubt
You can have a lot of one and very little of the other.
These two get used interchangeably, but they're not the same thing, and the difference matters for what actually needs to change.
Confidence is usually competence-based. It's built through doing something enough times to trust you can do it again, and it's often domain-specific, meaning you can feel very confident at work and much less so in relationships.
Self-worth is broader and less conditional. It's a baseline sense that you matter and are worthy of care, independent of performance. Someone can be highly competent and confident in specific areas while still carrying a low baseline sense of worth underneath it.
Building more skills or achievements can build confidence without touching self-worth at all, which is why some very accomplished people still struggle with feeling fundamentally not enough. Self-worth work tends to go further back than confidence work does.