He underestimated the amount of resistance he would get from his own Party. And he underestimated the pusillanimity and lethargy of Congress. Those guys don't want to do anything at all. Obama spoiled them. Now, they have forgotten what their job is supposed to be.
However, even though Trump is trying to slog through a waist high muddy swamp, he is still making forward progress.
The scariest thing in the world to the establishment of both Parties is that Trump might actually be successful. So, they are working together in concert to make sure this will never happen, regardless of the consequences to America or the American people.