Trusting willpower in the struggle with screen time is a lost battle — those apps are designed to hold your attention and they do the job well. The approach that works is this: making the phone just a little bit of a hassle to use. Even a small amount of friction breaks the "I opened it without noticing" loop.
You don't have to do all of the steps below at once; even two or three of them make a clear difference. The aim is not to give up the phone but to turn it back into a tool that is used when you want to.