The truth about telling the truth in the media is that much of what happens around politics and public officials is classified. No matter how many people witness a thing happening, not everyone who may watch or read reports about it is allowed the same level of access to information, and for good reason. Poltiicians, the President in particular, get caught between a rock in a hard place when reporters ask questions knowing that the Trump has been told by police and/or the military not to answer or not to give out all the details.
The fact that Donald Trump does not seem to differentiate between truth and lies is very disconcerting. I have seen many opinions written that perhaps Trump suffers from a narcissistic personality disorder and I'm inclined to believe that the disconnect that he seems to have between reality and fiction is the direct result of a clinical disorder.
Trump doesn't realize the different between the truth and a lie because he has convinced himself that certain things are true. Trump likes to believe his own narratives about things, because they make him feel like he's right with whatever he's doing. Trump should be more honest with himself, but he doesn't even realize what he's doing.
I really don't think Trump always realizes when he is lying. I think that he often receives false information from extreme sources, and believes it and shares it as truth. I also think the man is mentally imbalanced enough that he really does think his own crazy ideas are true and honest, even when he has just made them up in his own head.