Casual prediction: if Trump's poll numbers continue to spiral downward and senior staff leave, he will quit the race before Labor Day. Here's the deal as I see it: whatever his psychology, Trump is first and foremost an entrepreneur (and arbitrary, abusive boss) for whom every day is a new day and what you lose today you can win back tomorrow. Each day represents an opportunity, each day a new move in which anything is negotiable (real estate, the constitution, the law).
And if the business deal he's working on is not panning out (downward spiraling poll numbers, staff resignations, etc.), he may simply drop his bid, abandon his business partners (Republican Party), let go his temporary employees (supporters), and move on leaving everyone holding the bag. This is all the more likely in that Presidential elections come around only every four years--which is too long for an impatient businessman like Trump who wants to recoup his losses right away. If you lose your bid to be president, you really lose (sole exception: Nixon, but Nixon wasn't an entrepreneur).
I can't help thinking that the self-styled boss will never tolerate being deserted by staff or voters leading to his defeat by a woman no less. His view is the bully's and abusive boss' view of others: "Without me, you're nothing." His deepest fear, besides being simply ignored, is to become "nothing" instead. So he will pre-empt such a humiliation and fire voters and staff first by withdrawing his bid before Election Day, declare the elections "rigged," and return to the more exciting business of exploiting his commercial brand. Ever the entrepreneur, Trump always has more than one strategy in his back pocket (Plan B, Plan C, etc.).
The routine work of governing probably never interested him anyway (as some have already said); as President, to keep himself engaged, he would have to create the excitement of a new deal every day (suborning the other branches of government, deporting millions by executive order, unilaterally breaking treaties, declaring a national emergency, launching pre-emptive military attacks, etc. with none of the hypocrisy of the Bush administration) in which he could dominate others. But that requires a victory in November, which is 3 months away. He won't wait to find out.