Learning from the past
One of the main political lessons liberals continuously fail to absorb is that none of this is new, the world wasn't created yesterday --- and human nature is pretty predictable. Conservative argument keeps coming back, again and again and they succeed because liberals always want to "turn the page" and "let bygones be bygones" so we can "get things done." It doesn't work. They repeat these things like a mantra and they take on the patina of "truth" because well, everybody keeps saying it, it sounds familiar, it's always "out there." If liberals would resist the urge to forget everything they've learned about the right every few years they might be able to sustain the counterargument and have that take on the patina of truth. I'm pretty sure the last time that happened was Herbert Hoover.
Here's my simplified assessment of what's going on.
Some people are optimists. Some people are pessimists. Optimists look to the past and say, "We can make things better". Pessimists look to the past and say, "Things have been and always will be bad so we might as well take advantage of it." Optimists want to forget the past because it isn't how they want the world to be. They figure that dwelling on the past will just make it harder to change things. Pessimists become students of the past because all the awful things that have happened confirm their beliefs. It also appeases their consciences.
In this present time, Democrats are optimists. Republicans are pessimists.