Self Indulgent Teenage Shit. Can be used for people between the ages of 12 and early 20s when they are indulging in self-pity, overcomplicating their lives, constantly making statements that people don't realise how special they are, or otherwise speaking or behaving in an selfish, egocentric way. It is a synonym of "Drama Queen", and is commonly associated with Goths, Emos, Punks, Princesses, Fatties, and people with mental health issues such as depression and eating disorders.
Teenager: "I'm feeling really emotional today. Everything keeps going wrong. I feel like everybody in the world is out to get me, and I can't do anything about it." This person has SITS and should be told to get a life and harden up.
Moping around and refuses to acknowledge anybody, or is rude for no reason = two more examples of SITS.
Teenager complaining about parents piss them off and limit their independence, while said teenager refusing to accept any responsibility for their actions and expects their parents to still give them everything they "need" (want) = classic example of SITS.