[keeptrying]

Its not mental energy. Its actually habit. Work ethic is a habit... ie you do it WITHOUT thinking.

The only way to gain it is by working hard at anything. Preferably in a skill that you want to develop.

Practice practice practice. Going to the gym and staying healthy is important but have a closer look at Reid Hoffman, Mark Zuckerburg and Rex Ryan --- you wouldnt introduce them to someone who you'd like to impress with the benefits of gym-going.

Practice, practice, practice & dont give up.