From my experience, depression is something you have to sit still, hold on really tight and just let it pass, as you know your mood could change tomorrow.
Its hard because there isnt a 'cure,'.
All you want to do, is all you cant do, all you can do is the last thing you want and everything is backwards.
Try and concentrate hard on any positives you can think of.
It sounds like your best friend REALLY cares about you and tries to understand, thats so great, not everyone has that! She sounds like she is really there for you! x