This may sound really bad, but I personally find reading textbooks helps with motivating oneself to study harder and strive for better performance.
Professors/instructors rarely talk about real-life applications and examples during lectures, especially for the more technical courses such as accounting, mathematics and finance. Fortunately, decent textbooks usually include interesting case studies and real-life examples that serve as a good reminder of why you need to learn to understand and apply all these seemingly irrelevant theories and formulas.
I found lectures for economics to be particularly boring, so I took it upon myself to study for the course using the textbook. Every chapter in the textbook had a section where it would describe theories that were covered in the chapter and the real-life events that served as proofs or contradictions to those theories. Those sections helped me connect the technical things I was studying from the course to their use in computing relevant figures.
This strategy will work particularly well if you enjoy reading in general. If you don't find your textbooks interesting, you can always look up documentaries or articles that are related to the topics you have covered in your courses. My social studies teacher in high school showed a documentary for every chapter covered, and that made the otherwise not-so-interesting course quite engaging (for me, at least).
I hope this has helped. Good luck!
P.S. I should also mention that being depressed (as in clinically depressed, not just "sad" depressed) can cause you to lose motivation in things you used to enjoy. If you have been noticing unusual changes other than loss in motivation in past few months or even weeks, you might want to go see the school counsellor/psychiatrist. Some of the more significant changes that you should be attentive of include:
1) Sleeping habit - are you sleeping more or less than usual?
2) Diet - do you feel more or less hungry than usual? have you been eating more inconsistently than before?
3) Interests in other hobbies - are there any other things you used to enjoy that you don't enjoy any more?
4) Mood - have you been noticing that your mood is changing more drastically than usual? Do you go from neutral to really sad without any significant stimulus?
5) Ability to focus - do you find yourself becoming more distracted?
6) Energy - do you sometimes feel so weak that you can hardly get yourself out of bed?
There are many other symptoms to depression, and one person going through major depression may experience different effects than another. I think #2 and #3 are the biggest warning sign though, based on personal experience anyway. If you think you are going through major depression, the best thing is to seek professional help and do whatever is necessary to recover from it, even if it means temporary withdrawal from school. Major depression is not something you can naturally recover from... if anything, the more you keep telling yourself that you should recover from it without any professional help, the more depressed you will find yourself in your inability to make it go away.