Diaries were forbidden but soldiers kept them anyway.
The Imperial War museum has thousands.
Rob Naylor said he has one kept by his Grandad, so that was a silly thing to say Jim.

Letters were censored for locations and activities.
If a soldier was nervous of expressing disillusionment, he would just wait until he got leave.
He would not need to lie in his letters. Just not mention it.

Over 80% of soldiers made it home, so we can be sure what they thought.

So you are talking bollocks again.
All the historians are not fools who need you to explain their job to them.