@leaf True, but yes, it is chicken-and-egg. A lot of people who feel they are unworthy of being loved either had bad childhoods where they were neglected, or have had bad relationships that reinforced that feeling.
It's a little like chronic depression - it's not something everyone can just decide not to do anymore. It may have similar neurochemical changes since it is a form of chronic depression.
In fact, it's a bit related to impostor syndrome where you're convinced that you're successful but not through your own skills or agency - that's it's luck or you're BSing your way though it.
And it's almost impossible to tell someone with imposter's syndrome that they really ARE that good because that actually reinforces their belief that they aren't. I know that sounds paradoxical, but this isn't rational thought.
Similarly, testaments almost always have the opposite effect on people with any of these problems. It makes them feel that they're even MORE of a failure because the other person managed not to get into this state.
It's tricky.
You can't tell them to love themselves because they feel they are literally unworthy of love - even from themselves.
And no, I don't have a nice simple solution to it. I'm not even sure where to start since this is a 'prove a negative' problem in that to help someone with this condition, you have to convince them that they ARE worthy of being loved, but the very nature of the condition means that they'll see that as being manipulated by someone who wants something from them and will end up with their being abandoned.
See the problem?