To be completely honest bro, there's nothing wrong with weed when you're older. Say you go out fishing and camping with your mates, fine to have a bit if you're on a holiday. But you shouldn't be doing it at your age dude, i know it may sound corny, but the best medicine for depressing is laughter, and there's heaps of things and people on youtube and such that are willing to make you laugh.
Another problem is, if you keep smoking now and get addicted to it, if you ever wanted to get an apprenticeship once you're a bit older, you're fucked. No chance in hell they're going to hire some highschool kid who's high all the time.
Oh, and to answer your question. Tell her honestly why you do it. Don't run off on a random tangent trying to explain how good it is, because as much as you want to believe that and trust the 'scientists' who proved it, scientists also probably proved that fucking animals is good for your well being or some shit. Not everything said by a 'scientist' is actually scientifically proven. For one, you're inhaling smoke which isn't good for your lungs, and to be doing a drug like that at such a young age which makes you feel disorientated, you could be just fucking with your brain and that's not good either.
TL;DR There's no reason you have to do it, many other things can help depression. Tell her straightforward why you feel you need to do it, and don't try to prove that it's 'good' to her.