Lost motivation and depressed

Something that can help is to set realistic long-term goals. Often, when students feel they're falling behind, they'll decide that they're going to try really hard and do several hours of intense work a day. Since this isn't a reasonable goal for most people - especially students who are struggling - they end up giving up.

A better way to approach it is to set small goals that you must complete each day. For example, you must do one hour of work each evening. Then, reward yourself for anything you do above what you said you must do. This way, you won't be left feeling very guilty after 'bad' days, because you can still probably manage to do your base goal, and it becomes easier to be consistently motivated.

As the other poster mentioned, it's also important to work efficiently. Look at your current study methods and try to be reasonable about what's working. Spending time copying up notes or copying things from a textbook isn't very useful but takes a lot of time. Doing past-paper questions and breaking down topics into mindmaps take less time and are more effective.

If you haven't already, speak to your form tutor / any other pastoral staff at school. They may be able to offer some support as well.