Hi Mosp. I keep diaries. They help me process thoughts. My first diary was when I was 6 years old. It was an A4 weekly planner from my father's business. There are lots of entries like "today we did gym" and "I played with Claire and drank juice". Then one day, "Mummy punched me in the tummy". I too have wondered if I exaggerated that. Maybe she pushed me away and it wasn't my stomach she touched, for example. I've thought of HUNDREDS of possibilities over the years (she probably punched me, but not really, really hard). What I think of it is that a) whatever happened, it was important to me b) as a 6 year old, I understood it to be a punch and not a mistake c) I know with hindsight that I wasn't lying in my diary and d) I put it in context with my mother's behaviour to know that it may have been an exaggeration, but that would only be a small one and have been because I was really upset about it. So, it was an exaggeration of the fact, but a 100% honest reflection of how I felt about it in totality.
People do lie in diaries. They do, especially if they know someone might read it. Mostly though, they don't, they just get whatever it is out, for whatever reason. It is unlikely that in a meticulous phase of life that someone would write a traumatic event down in detail and lie, or even exaggerate to a point where the truth is distorted however. There would be little to gain from that.
I too went through a traumatic event at around the same age as you. I wrote it down, in detail. I sent it to a friend, however, I didn't put it in my diary. I have no idea why. I also wrote that she was to destroy the letter after reading it and tell no one. She did destroy it, but told one other close friend. I wish I'd not asked her to destroy it, because it would help a lot to know the details I've blocked out now and i no longer know her well enough to ask. I'm sure my memory has twisted parts too. That's what often happens with memory though.
I can't imagine having even my loveliest therapist read my diary, but fwiw, I think you've done a brave thing that is incredibly difficult now, but will help in the long term.