Depression leads to addiction or the other way around ?

I am a 18 European Caucasian male.

I spend 5-16 hours a day using my PC to navigate the internet.

I know I have an addiction to the use of computers but it easily goes away after I spend more than 24 hours away from it.

I'm also attending college and I've dropped many classes for no apparent reason (I lost interest).

I've had a depressive behavior for years and I've always kept it under control.

I think I'm losing interest in some parts of my life (although I attend 3 times a week the college's music group meetings).

I don't play online games anymore and I don't think I'm ditching classes simply because I want to be on the computer (I don't think about the pc a lot when I'm in classes, etc).

I was wondering if an addiction like this could lead to depression and/or the depression(s) could lead to addiction.

I feel sad and melancholic (have lack of sleep / trying NOT to sleep for no apparent reason) every other day.

I keep away from any addictive substance (nicotine+) because I know it would be even worse than my current situation.

I would like to hear a doctor's opinion on this.

Thank you,
Hugo A.