Here's my take.
It's not the US's ugly issues we are really talking about. It's the entire world.
The root cause of them all is disinformation.
If we look around the world there are numerous countries where fascist, authoritarian, populist ideologies are holding sway, the US is the largest and easiest to see but it's happening throughout Europe too and has been happening for years in many other countries.
People are suckers for a good story and will convince themselves that X is true despite all evidence to the contrary (see religion) and nowadays we have a way of spreading information rapidly. The internet in general and social media in particular can rapidly disseminate information to anyone who cares to receive it, regardless of how true it is.
Lots of people regard sites like Breitbart (other wingnut "news" sites of all political stripes are available) as credible news sources, when they are anything but. Most of the wingnut news paint the mainstream news as biased and slow and not credible, and people believe them.
"Never believe anything you read in the papers" was an old refrain the same is ever more true of things you read on the internet.
Whatever you think of MSM they are typically regarded by the law as publishing information, and as such they face legal consequences if they make up stuff. Blogs and forums and facebook and twitter notsomuch.
Some bad actors have seen how this works and are weaponising it to a) make a pile of money and b) spread their particular ideology as much as they can. States like Russia and likely others spread many false stories in an effort to destabilise people they regard as their enemies. Some people just like spreading rubbish just for the lolz. Some honestly believe what they are spreading around.
The other facet to this is big data. Google came up with a genius algorithm to place adverts. They wanted to find a way to make money. They provided a very high quality product (internet search) free to use to the world, and they settled on advertising. They placed curated adverts in the search results tailored to individual users. As a result f how good they were at doing that adverts placed with them got more clicks and sold more things and made them piles of money. They keep iterating on and improving their algorithms.
It turns out though that those same algorithms can be used to place "alternative facts" with low educated, low turnout voters. We've seen the rise of big firms like Cambridge Analytica who are *damn* good at what they do. Dominic Cummings pulled this trick over Brexit. Steve Bannon did it with Breitbart and many others.
"A lie has got halfway round the world before the truth even has it's boots on" someone once said.
We need to change things so that we argue both sides of an issue based off of the same set of facts. We need to marginalise conspiracy theories and wingnut news again. We need to get back to healthy debate over issues we face, and how to solve them.
We need to co-operate with and compromise with people who hold opposing views to move things forward.
We need to redistribute wealth, so that the top 1% of the top 1% don't hoard everything and everyone in the world can afford basic human necessities like clean water, food and healthcare.
We need to stop releasing CO2 into the air and find ways to mitigate the damage we've already done to the planet.
We need to build infrastructure to colonise other worlds before another asteroid hits.
We need to do a lot of things. Though it starts with being less partisan and talking with people of opposing viewpoints.
It's not just the US. It's the whole world, though like it or not the US tends to drive a lot of policy in other countries too, so having a POTUS who isn't an unwitting puppet of wingnut ideologues is a good start for now.