Paradoxically, the negative thoughts won't have a grip on you if you stop trying to dig them out but accept and embrace them instead. The more you beat yourself up for having those thoughts, the more they will persist. Do you know the expression "what you resist persists"?

Frankly, I think, you'd be more helped by the therapist who would help you understand the origins of your negative thoughts and would be more accepting of you and where you are at the moment instead of instructing you to do the impossible and implicitly shaming you for not being able to do that. It seems like your current therapist is making you feel crappy about yourself, and there is nothing therapeutic about that, even if it's done unintentionally.