re: Why is this choice so damn difficult for Democrats?

They don’t want to be perceived as backpedaling on previous stances they’ve taken re: open borders/ immigration.
To do so would be, to them, tantamount to admitting that they were wrong.
It’s a pride thing (not the rainbow kind).
Rather than cutting their losses and recognizing that most Americans are hugely in favor of a massive immigration overhaul, their party is going to take a huge hit.