As mentioned elsewhere, interfaces really a form of inheritance. They just generally have no implementation code in them.
I also very much understand the thrashing about that occurs when trying to abstract and inherit and compose my classes while making progress on a new idea. I think I early on solved it by just ignore trying to do everything The Right Way and write code that works to the solution. Then as the code evolved I could easily see where code was duplicated and begin grasping where things should be sharing code in some way. It greatly improved my efficiency AND made my code cleaner in the long run.
I'm not saying I'm smart, but it's funny now that it at least seems like the whole "modern approach" is anti-inheritance and more about "interfaces" and composition, which essentially is what I did anyway because inheritance OOP got me nowhere.