There are only two possible explanations. First, he's suffering from dementia or mental illness and is unable to tell fact from fiction. Or, two, he just makes things up on the spot, or repeats things he's made up in the past.
I think the second explanation is far more likely. Quite simply, he lies habitually because he knows he can get away with it in front of his cultist-like followers.
Increasingly, however, Americans are tiring of Trump. Eight of the most important political polls, all conducted within the last week, showed support for the President is dropping to new levels (details here).
That's very bad news for Republicans less than two months before midterm elections. But we all have to carry through on this momentum and vote.
Opposing Trump is not enough. We have to do it at the ballot box in eight weeks.