Could I be suffering from depression?

I am not sure what could be possibly wrong with me. So this month I've written down everything that has concerned me. It is a long list, but I am going to point out the key points. 1. I am pessimistic! My friend told me I can not keep a positive attitude. At first I didn't agree with him but as I look back,... show more I am not sure what could be possibly wrong with me. So this month I've written down everything that has concerned me. It is a long list, but I am going to point out the key points.
1. I am pessimistic! My friend told me I can not keep a positive attitude. At first I didn't agree with him but as I look back, he is actually right.
2. I feel sad. This used to only happen when it was late at night. Now it has gotten worst.
3. Isolation. I dont mind being around people like my friends and family, but I realize that I am very quiet and I cant seem to fit in like I used too.
4. I have a bad attitude/irritable. Anything can set me off. I lashed out at many of my friends unintentionally when they swear they didn't mean it and I hope I can change that.
5. Unmotivated- I used to be so excited to see where life is going to take me, unfortunately I care, but Im just not interested in anything anymore, and I dont know if im going down a positive road.

6. I think of dying very often, Its either me or a family member I never had these thoughts until a while ago.

I am 16 years old. Thanks!
Update: I forgot to add that I never did good in school, even though I want too. I ended the school year with D's and a B, but when the school year started I had A's and B's. I hope I can figure out whats wrong.
Update 2: My parents also noticed that I look sad, sometimes I just dont notice that im sad. sometimes I just feel alone, but never sad.
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