Google have probably removed the option to get back to the old layout. It's the way that they normally do things and was predicted by sites such as this one back in April:
https://9to5google.com/2018/04/25/how-to-switch-classic-gmail-android-basics/
(Quote: "Just like when Google rolled out a new look for Google Calendar, the ability to change back to the classic design will most likely go away in a couple of months. So while you might prefer the classic Gmail look, you’ll eventually have to switch and use the new design").
The recent comments on that thread also seem to suggest that you're stuck with the new design.
All I can suggest is either:
(a) using a dedicated email app, rather than webmail, which gives you far greater control over the layout of things (
https://www.thunderbird.net/en-GB/ ) ; or
(b) switching to a different email service, with a far more intuitive layout (
https://www.gmx.co.uk/ ). If you do so, you'll still be able to use your Gmail address, simply by using the 'mail collector' facility within GMX.