Trump is the archetypal narcissist and in good company along with most autocratic dictators that have ever existed. Putin, Erdogan, any number of leaders of former Russian areas, most South American and African countries, even this country. Politics or other forms of leadership breeds people who think highly of themselves - it is a real chicken and egg scenario. Confident people are rarely wrong (in their view). The masses should be grateful that he is only going to be President for one term or possibly two because the system is designed to ensure nobody can have an indefinite monopoly (that's a good thing) - unless he does a Putin, that is.
His basic problem is that he seems to be in permanent campaign mode - and we all know that politicians say what they feel they need to say to be elected. Just treat it like any other soap opera on TV, i.e. something you switch off when the intro music comes on. We all have a big friendly off button on the remote.