Long term goals are great motivators. But the way you get there is by adding up enough good, productive individual days. And that means focusing on short term goals, too.
For students heading back to school today, just worry about having a good, productive day you can be proud of. Pay attention in class. Ask questions. Use your homework time as a time to really learn the material rather than just get it done as quickly as possible. Bring some effort and enthusiasm to your activities. Smile. Be nice to people (including your parents, your teachers and your counselor). When you finish the day, be honest with yourself about whether or not you really did your best in all those areas. If you didn’t, don’t beat yourself up. Just resolve to be better the next day.
Add up enough individual days like that, and you're bound to be happy with the outcome.
And while you're at it, remember that while nobody said reaching any of your goals would be easy, there's no law that says you have to be miserable to be successful. Make sure that the pursuit of both your short term and long term goals actually makes you happy, too.