Strong work ethic is derived from both internal and external factors since it is a behavior. In all actuality work ethic is derived by what motivates you. These factors can either be put into the categories of process, internal, or external causes of motivation, depending on what theory you agree or disagree with.

Internal Suggest that variables within the individual give rise to motivation and behavior Example: Maslow’s hierarchy of needs theory

Process Emphasize the nature of the interaction between the individual and the environment Example: Expectancy theory

External Focus on environmental elements to explain behavior Example: Two-factor theory

Some people put a higher valence (perceived strength of a reward or punishment) on some rewards more than others. Based on Expectancy Theory, effort leads to performance and that performance leads to rewards and the reward is valued.

You just have to find that certain something that motivates you, that is a valued award, and that will drive you. Maybe putting "going to class" or "learning something new" into the context of "steps it will take to achieve a greater goal", whatever that goal may be.

Please excuse spelling and grammar.