No, T doesn't need to know everything, only what you need to tell him. None of my Ts knew everything about me or my life and it would be impossible to give the whole story in any case. I tell my T what I feel in my heart is necessary to say for my healing or to help me lead a better/healthier life.

But I agree with Can't Explain completely that often it's the things we can't say that are the most important. Even when we convince ourselves that it's not important. Often its those issues that have been there most of the time but unspeakable to anyone.