I used a flip phone for a year after having had a smartphone for a decade and I felt similarly to you when I first went back to my smartphone. It was overstimulating and I hated it. I have always used an iPhone, though, and I find that they're a significantly better user experience than Android so I didn't notice the UX problems you described. More like the entire thing was just TOO MUCH COMING AT ME ALL THE TIME AHHHHHH! After a few months I was back to being addicted and it all felt normal again. I miss my flip phone a lot but life without a smartphone was just too annoying.
Basically, I despise my smartphone, but I'm still addicted to it. I use it even when I don't want to and when it prevents me from doing other things I'd rather be doing. I try to stop and eventually I always give in. It's a legitimate behavioral addiction and I believe the problem is both the design of specific apps and properties inherent to the device itself.