Identify a career field or goal
Having something you're working toward always helps you keep on track and helps you keep hope alive when things aren't going as well as you'd like. With a career goal in mind, when a particular class gets hard or gets boring, you have something to hang onto, "I can get through this short-term problem because when I'm done I'll be one step closer to my goal."
Yes, many students change majors many times during their college years, but usually they aren't 180-degree changes. They may shift to a related field, like from biology to botany or from history to anthropology. By the end of high school, with some careful thought and with some guidance, most students can narrow down a career field in which they can see possibilities for their future and begin to set goals.