> "I'm valuable just because I exist?"
That's a start. What missguided thinking would suggest otherwise?
But that doesn't mean, that you are valuable (able to provide value) to someone else.
However, the point about self worth, I like to make is: self worth starts with you - based on experience, and obviously supported by a feedback loop you gain confidence in what you're good at, and what not.
So, the best way to gain self worth is by gaining confidence, is by gaining experience.