Here's something I did to get past my own wall two months ago. I told myself to shut up, set aside some time each day (after I started working it was a lot easier. The only time I had was after work hours till 10 30 in the night) and then follow these steps.
1) make sure you are fresh, preferably having had shower 2) phones, distractable media all shut off and put away. block everything except your resources on the net if you have to 3) put down what you are going to do either on a small A4 size like whiteboard, or an A4 drawing paper (blank white is best to focus) When putting down the stuff do it this way a) write down the features you want to implement in your current development cycle b) draw up the screen designs if you need to c) think through the logic and put down what's needed.
Get to Work!
Methodologies are there for a reason. They do actually work. Btw, before starting. Make sure you have a clean overall view of the project as well. Not just the 'pitch'. Put down your main use cases and the features for each use case there (two levels max you can elaborate during the main cycles).
What's wrong with you is not lack of motivation. It's a lack of focus. I've dealt with it when learning languages too. I'd spend hours reading the material and come away with nothing. You need to know what you will achieve and that within a practical time frame. If you are having a really bad case of not being able to focus, I suggest a maximum time frame of two days. I currently work on a functionality a week/fortnight basis.
Good luck :)