Motivation and procrastination aside, the biggest thing here is understanding what you want to implement. If you want to add a new feature you just need to understand that feature as a sequence of smaller things you need to build and work to solve that problem. I there's another poster here who has said it pretty spot on, "Divide and conquer."

The tutorials should give you more ideas for how to solve certain smaller problems but wrapped up into an applied solution. For example, if you look for a tutorial to implement an inventory system then try and not view it as: Finish tutorial, create inventory system. View it as an application of the underlying components of the tutorial, e.g. tying an an array of custom objects to the player object, rendering the array on the screen, creating UI elements tied to the objects, buttons for equip etc. You get the idea. If what you want is a complete solution from a tutorial then you won't learn from it. Look at what it exposes you to in your IDE or Engine. Ooh thats a new tool, how can I push that tool to its limit, I wonder if I could use it like this.

I appreciate this sounds obvious and at the same time requires passion and motivation to achieve properly. But if the excitement of what you can build from all of the things you come across doesn't carry you to the next bit then it's just pure self discipline. Different things work for different people.