He's dealing with a political reality that is entirely out of his control. He must walk a fine line between fulfilling his promises and alienating a Republican Party leadership whose backing he needs to run again in 2020.
Eh, yes. But it's also a reality that President Trump was well aware of the political situation/climate when he was running for office and essentially doubled down on making building a border wall and immigration an essential part of his campaign.
President Trump will be measured by many people on how many miles of border wall were built. It's a pledge (promise?) he made during the campaign and also once he was in office. For these people, immigration is the singular defining issue of the Trump presidency.