I'm always tired and if I sit too long I fall asleep. I can't get motivated either. How can I change this?

Sleepiness. Sleepiness is a symptom. If you are spending enough time sleeping and in bed then you most likely have a sleep disorder such as sleep apnea. Discuss this with your doctor. Your symptoms can be improved.
See your doctor. You could be fatigued due to hard work and poor sleeping habits. Depression, thyroid abnormalities, anemia of any number of types, sleep apnea or other causes of chronic insomnia are possible as well. The best option is to talk to your physician and get a complete workup as soon as possible. If no physical abnormalities can be identified a visit to a counselor or psychiatrist would be a good idea.