internal motivation means that someones acts without external stimuli, their drive comes from within, its internal. External motivation means that an external stimuli is used to make someone act. I.e. a monetary reward, or validation etc. When someone is internally motivated, they can have a stable state. When external motivation is introduced, it can replace the internal motivation and will. Now what happens when you then lose the external motivation, the external stimuli again? The internal motivation is gone and this means all motivation is gone, the act stops.

On top of this, some people say motivation is cheap, discipline is what matters.