There are plenty of 'more successful Confederacy' timelines, but how can things go badly wrong if the Civil War ends as OTL? I'm not thinking of outright dystopia but rather something that ends with the 20th C US being about the equivalent of Brazil or Argentina.
Sticking near the ACW POD, the three biggest things would be economic collapse, limiting US expansion, or limits to political/military influence.
Lots of possibilities for economics -- earlier, worse depression, etc.
As far as expansion:
- AK. US doesn't buy AK and Russia builds it up more (or the UK gets it)
- HI. Things evolve differently so the US has less influence, the US isn't able to lease bases, and other powers (UK mainly) get involved so HI remains independent.
- West. Native American conflicts in the West are worse, go on longer, and aren't settled with similar treaties. So, many of the (post ACW) states aren't admitted until much later and some native nations maybe even retain some independence.
- Spanish possessions. The Spanish-American War never happens or goes poorly for the US. The US doesn't get Guam, PI, and Puerto Rico. Cuba becomes an independent nation and resists US influence.
- Panama Canal. The French, or others, build the canal instead of US and exert their influence there.
In general, US interests in the Pacific are reduced and influence dramatically curtailed.
For political/military influence, different US presidencies/congresses could focus differently, screw the US up diplomatically, turn the US more strongly isolationist, and keep the military much weaker. In these scenarios, the US is lessened as a power, might never get involved in WW1, etc.