You can say all of these hollow-sounding things, but you know, deep down, that it's just talk. Not real talk, just "talk". The kind of talk we can be having for 30 pages without any real results. This is politics we are talking about, not some guy promising you a reward, and failing to deliver. There is more at play here than just Trump's will to see things through, a world where circumstances are ever changing, a government where he will be met with opposition against any of his proposed changes.
People are clever enough to understand the difference, which is why not buying a politician's promises, literally, but rather taking them into account while keeping political realities in mind, is basically a starting point to becoming an intelligent voter.
"Making Mexico pay for it" was a sound-byte, not something that people took seriously, except in election-time fervor, which in America tends to be especially zealous. Nobody in Trump's camp will give a damn if he doesn't make Mexico literally pay for it. The people that pretend to care, are those who can politically gain from Trump failing to live up to his promises in literal detail. His "base" are far more willing to make compromises as long as he is esteemed to be achieving enough things. Which is what has happened with all prior American presidents, anyway. None of them have lived up to each of their promises, some barely lived up to any, their worth was gauged on how big of a percentage they were able to accomplish, and how well they were able to atleast see their will through in compromises, which is ultimately the real "skin" in the game, how capable you were at carving out compromises, with your political foes. The same will happen with Trump, he will be judged "relatively" rather than "absolutely", once all is said and done.