How much transparent can I be with my manager?

I am a new grad at a unicorn, and like many other new grads I am also struggling at my job and facing severe impostor syndrome / stress / anxiety. All other employees are very highly capable and the bar here seems to be very high. I was curious how much is it okay to share with my manager? Can I be 100% transparent? Or do I have to be strategic about how I communicate? I am afraid if I am being too open about how "unskilled" I feel I am, he might see me as less capable and not deserving of more impactful projects.

Speaking about strategic communication, I realised I struggle with it in general. I am a bit too authentic and direct, both in daily life and professional life. How does one learn the art of strategic / diplomatic communication?