Is it normal that i smile when i'm sad?

At school, I'm unbelievably depressed. I hear about all these quotes and posts about people masking their unhappiness with a smile, and I can sort of relate. However, I smile unintentionally. There's this girl in my class that always insults me, telling me she loathes me. Whenever she tells me I'm stupid or a freak, I feel really hurt but my natural response is to smile. Also, we act like best friends. Better than my actual best friend, to be honest. Everyone I know there thinks I'm a happy, positive girl, always smiling, and my expression at school is always like that, but my thoughts are so apathetic, logical and unusually pessimistic.
At home, where I'm happiest, I always look sad and hostile. I never come out of my room and I spend my day on my computer. I get either really aggressive or really depressed for no reason. On the inside, though, I feel really ecstatic and hyper. Sometimes my actual mood will leak into my expression, so I end up switching from sad to happy really quickly.
Lately, I've been trying to ignore my best friend, whom I depend on too much, because she clearly doesn't want me around. My classmates have noticed that I've changed a lot, and the girls I rarely talk to are genuinely worried about me, asking why I'm sad. My best friend doesn't even notice how upset I am, and that I'm trying to avoid her especially. The fact that she doesn't care makes me really upset. I constantly contemplate self-harm and suicide, even though I despise the very idea. I think I have Dissosiative Personality Disorder.
I recently realised that all my problems have happened because I smile when I'm upset...
Is all of this normal?