How does one create sustained motivation?
The starting point is to have a powerful purpose or motive for doing something worthwhile. If the motive is not powerful, motivation will also be lacking.
Ask the question why am I doing what I am doing? What is the purpose or motive? Am I driven to do this for an external gain or is there an inner drive?
Usually it is the inner drive that creates the motivation. External reasons are fine but not enough to create sustained motivation.
To be motivated, ensure that the outcome that you are aspiring for has great meaning for you.