People who really hate themselves have crossed into depression. And as I've said before, depression is an all-consuming negativity. Believing that you are unworthy of happiness is a serious issue, and no single conversation will magically make it go away.
You can't feel loved if you hate yourself. Characters need to have significant experiences and major revelations before they can truly accept another person's love. Otherwise, the all-encompassing negativity of their depression would convince them that nobody else really likes them. And that is the ultimate question: how do you convince a negative person that they do deserve love?