People should never self-diagnose, especially if they’re teenagers. There’s a reason why it’s much harder to professionally diagnose a teenager than an adult: teenagers are moody, anxious, unhappy and impulsive by nature. If you’re not a professional, you cannot diagnose yourself objectively. You will look at a list of symptoms and think “hey, that sounds just like me, I guess I have Borderline personality disorder, that explains everything!”.
And what’s wrong with that? Well, for starters, you won’t understand how serious mental illnesses are. Just because you are moody and irritable doesn’t mean you’re sick. Google the symptoms for Borderline personality disorder. “Problems with regulating emotions and thoughts, impulsive behavior, unstable relationships with people, distorted and unstable self-image" etc. Now, what teenager doesn’t fit into those?
I’ve seen people self-diagnose and use their “diagnosis” as an excuse to not do things way too many times. Can’t stand school, I have Chronic fatigue syndrome. Got denied a job at a job interview, it’s because I have autism. Go to sleep at 3am every night, I have insomnia.
There is nothing wrong with suspecting you might have a mental illness and checking symptoms, etc, but you are not a professional and you cannot objectively diagnose yourself. If you suspect you are mentally ill, GO TO A PROFESSIONAL. If you can’t afford that, don’t, but don’t go around telling people you have autism or OCD or bipolar disorder when you haven’t been diagnosed. You can say “I have compulsive thoughts” or “I am depressed” or “I have trouble making friends”, or even “I think I might have schizophrenia” without diagnosing yourself.
If and when you do see a professional, tell them you’ve thought about it and you think you might have OCD or schizophrenia or whatever. It will help them.
If you know someone, anyone, who has studied psychology, ask them to take a look. Really, anyone, psychology student or not, would be better at diagnosing you than you yourself, because once again, you can’t look at yourself objectively. You may think that your mood swings are abnormal but people around you probably don’t. Ask them, “Do I have abnormal mood swings?” It’s not that hard.