I would say focus less on the OO programming and focus more on the OO design. Grab a paper and a pencil (or maybe a UML modelling tool), and get away from the screen.
By practicing how to design a system, you’ll start to get a natural feel for object relationships. Code is just a by-product of design. Draw diagrams and model your application in a purely non-code form. What are the relationships? How do your models interact? Don’t even think about the code.
Once you’ve spent time designing… then translate it to code. You’ll be surprised at just how quickly the code can be written from a good OO design.
After a lot of design practice, you’ll start seeing common areas that can be modularized or abstracted out, and you’ll see an improvement in both your designs and your code.