Hi I can totally relate to what ur saying. U r definitely NOT being selfish, ur welfare is more important than anything! I'm sixteen and was diagnosed with clinical depression, but it's gradually improving although it's slow (i never thought it would though). You need to tackle what is the cause(or adding) to ur depression, if it's school then u should change school, as it's not like u only have a few months left, u have 2 years or so. i was actually advised to leave school even though i was in my last year, ur welfare is way more important than education. I was close to suicide at that point, my "friends" were not surportive at all, most people just don't understand it.
But please don't harm urself, i know exactly how u feel. Just remember u r living ur live for u, no one else. it does not matter what people think of you, it only maters what u think of ur self. You need to do stuff that will improve ur live and make u happy. i hope that helped