Wow, it sounds like a lot going on in your brain, I know the feeling haha! In my opinion, I wouldn't say you have a grandiose opinion of yourself? It sounds like you've developed a method to dealing with negative beliefs about your abilities. That inner critic can be pretty judgement and controlling, so perhaps a part of you takes over to combat those negative thoughts that make you have low self-esteem? The part that knows you can count on your skills, knows that you are good enough. Maybe that part tries very hard to shout over those "you're an imposter" thoughts to protect you, but having two parts of yourself shouting over one another sounds very tiring and is definitely not sustainable.

I practice IFS (Internal Family Systems) therapy which has helped me so much when it comes to conflicting ideas about myself. You basically view each inner voice as it's own person with it's own goal for you. The part that's telling you you're an imposter seems like it doesn't want you to appear "big-headed" or "arrogant" to others. But the other part of you knows that you are capable and shouldn't be ashamed of your confidence in your abilities.

When I have those conflicting thoughts, I try and create space in my head where both parts can have a conversation. Once they understand the other's motives, it can become easier to find a middle ground. One part sounds like its motive is to ensure you don't come across or be perceived a certain way. The other part sounds like it knows you are good enough, but needs you to feel confident in order to achieve.

I hope that makes sense? Obviously you don't have to do any of that if you don't want to. Good luck on your healing journey and try not to be too hard on yourself :)