So I decided to make my first app. All went to hell.

I recommend that you look at the code of other people who've written Android apps -- apps that don't have the problems you're experiencing.

There's plenty of open-source Android code available for examination. Many of them support multiple resolutions and most of them don't have the problems you're struggling with.

An alternative is to slow down and learn how to write Android apps at a normal pace, one step at a time -- don't be in such a rush to build an app that you don't fully understand.