People do not have a problem with work ethic, people have a problem with wanting. Anyone will work hard for what they want–the trick is getting to the point where you really want it.
You can illustrate this point however you like: immigrants moving from a home in the country to the city, where they work a terrible job for 14 hours a day just to survive; Americans working 60 hours a week and foregoing vacation leave for the Dream; the Israelites wishing to return to Egypt after getting tired of the wilderness. That’s right–people will even enslave themselves to get what they want.
So the issue is not mustering up your work ethic, or building up you courage; the issue is developing your tastes to want what you believe you should. The issue is developing a character that is inclined toward God. Honestly.
The most important commandment really is that straightforward:
“Love the LORD your God with all your heart and with all your soul and with all your strength.” -Deuteronomy 6:5
It’s the only solution for legalism. It’s the only enabler of sanctification. It’s the only sustainable driving force for your faith.
And just like learning to love any other person, it has to develop, over time, through shared experience.