Depression and melatonin

So I've been diagnosed with depression and anxiety, and a year ago I started taking melatonin pills every night because I couldn't sleep (I always took 6mg). I had a break during Summer when I overdosed on pain killers and my kidneys were in danger so I couldn't use any meds (and my parents hid everything). Now I can't sleep without melatonin again. I do it during weekends and I sleep horribly but normally I take the 6mg. Lately I've been taking 10-15mg which is clearly way over the amount I've been given in my prescription, and I'm only 15. It makes me always feel weird, like the next day I feel tired and really sick and I have to drink coffee to make the feeling go away or I'll pass out in school. And when I take too much of those, I start feeling a lil weird before I fall asleep, like so relaxed that it feels like my soul left my body and i'm not sure if i'm imagining it or if that is possible. Does anyone know if there's any issues that could follow up because of all this, I mean, it seems ridiculous because it's just melatonin but i'm getting worried. Thanks