Do I have a mental illness?

For the past 6 months I have been feeling really differently, I have severe mood swings i dont notice until friends or relatives tell me it, I get really angry at little things and start yelling uncontrollably, then another day I am all depressed and sad and then I am only happy if good things happen to me. I cannot control my emotions most the time, I've not been going to school the past couple of weeks and am resorting to Alcahol and sometimes even take my brothers concerta medication. I lose interest in the things i like then regain interest another day. I feel an enourmous amount of mental pressure sometimes and think that I am guilty. I sometimes hear weird noises from various directions, and then look in the direction to see nothing there. Sometimes i can also see very short flashes in the corner of my eyes and make it out to be cockroaches or spiders. Sometimes at night when I'm in bed i think there are insects crawling under my quilt or pillow and i get up lots of times to turn the light on and find nothing there. Sometimes i sleep like a baby. I sometimes get paranoid on little things as well. I am an 18 year old male, and i know that was a lot of detail, but would like an idea of what's wrong with me? thankyou