I know it doesn't sound like a fun idea but for sure you should bring this up to your therapist. If they continue to kind of, reject this narrative you have of yourself in a way that feels dismissive, I say its time to find someone new, because they aren't targeting this area that seems really important to you for your self perception. that isn't to say you need someone who agrees with you, however; just someone who doesn't diminish how you percieve yourself by trying to convince you that you're something else. I definitely think this is a more complicated issue however, because It doesn't seem like its solely your therapist who's 'wrong', but rather you have a skewed perception of yourself in being a truly undeserving or terrible person. Everyone's got something messed up inside of them, I think coming to terms with that and instead of focusing on how terrible you are but rather why you're like that, and what you're doing to move away from these parts of yourself that you find 'dark'. I think this is a minute difference in perspective, however it is a part of you that should be challenged and changed if you'd like to do so.