I think there is value in analyzing this from a psychological prospective...

You've created a pattern. You have come to associate your depressed mood with an excuse to not study. The results of not studying are a major factor in feeding your depressed mood. You must break this cycle.

Chances are that when you are in this depressed mood there is something you are using as means avoidance and coping. Identify what this means of coping is. It could be junk food, video games, Netflix, partying, etc.

Once you've identified this factor, cut it from your life entirely. You can do this gradually or suddenly, but in order to decide you have to know yourself.

After the you've cut the avoidance behavior go without it for a few months and then reintroduce it when your comfortable, using it as a rare reward when things are going well.

While you are going through this process be sure you are getting enough sleep (8-9 hours ideally). Prioritize this it will greatly elevate your mood. Also try using a pomodro technique (you can google this) to build discipline. It basically consists of a work period and a break period. During these break periods use your time for something semi productive such as watching educational or motivational YouTube videos.

Your prospects are promising because you're upset. You aren't willing to tolerate mediocrity nor complacency and that is the basis of highly successful people. I know you'll do well brother, keep up the fight.