Am I TOO pessimistic?

What are some signs you're too pessimistic? I just realized I think I'm the most pessimistic person in the world. Today I was sitting beside one of my friends and I was like 'you're always in a good mood, so friendly, always smiling' then *BAM* it hit me...maybe I'm just always in a bad mood! I see other people at school and everywhere who are so friendly and outgoing. Everyone seems so positive and happy for the most part...we all have our days when we're down and negative....but I feel like that's my default I'm ALWAYS negative and expecting people to not like me, hate me, think I'm ugly. Everyone has friends and I'm a loner... At school (or any public place for that matter) I am EXTREMELY self conscious, scared, awkward, quiet, tense, uncomfortable, hardly smile (and it doesn't help that I have a naturally sad/mean looking face because of my braves and wonky teeth)...In class, I just sit there, don't talk to people and when I do I act kinda awkward because I think they're judging my looks. I always have my head down, or I'll be look around the classroom to see if someones looking at my mouth or something and thinking 'her mouth looks weird or ew her hair is ugly', and sometimes I don't even look at people because I'm scared to read their facial expression... I ignore my 'friends' and people who are so nice to me because I feel like I have an impending doom eventually they'll find something about me they don't like, or they'll realize how awkward I am, notice my ugly flaws...I hardly say hi to my friends in the halls because I think they won't hear me anyways and I'll just feel stupid (it's happened lots of time before). Like one day I was talking to my friend just fine, had a great conversation...Then the next day she had walked into class and sat down, smiled at me, and was staring at me for about 15 seconds, waiting for me to finish talking to the person I was talking to. But I pretended not to notice because I HATE when people stare at me...and I felt ugly so I didn't want to look at her, then when I finished talking to the girl, she was still staring, but I was pretending to read something on this paper I was holding. Then she just sighed and turned around, stayed quiet the rest of class... I don't mean to be rude...but I just felt ugly and vulnerable because she was staring at me. I remember even as a kid...this one guy used to stare in me in class all the time and it annoyed me so bad because I thought he was mocking me and thought I was ugly or one day I was like 'stop staring at me you ugly freak' and to this day I feel SO bad...because he's a really nice guy. I was like 11 then...I'm 17 now. A lot of people stare at me, ask me why I'm so shy...I am so shy because I always assume people think I'm ugly and weird...then come to find out people tell me they think I'm 'really pretty' and 'very beautiful'. And the stares people give me, I assume they just hate my guts, think I'm ugly, and that I'm the weirdest loser they've ever met... but then later on they'll admit that 'I seem cool but I seem stuck up or intimidating because I'm so quiet'. I have no friends because I'm so uncomfortable around people, always expecting them to judge/hate me. I've misunderstood so many people and would just assume they hate me...but turns out they don't. At one point I was CERTAIN my history teacher hated me and I would go home to my mom asking her if there was something wrong with me because I feel like every living thing hates me... my history teacher would never make eye contact with me, always ignored me...but it turns out he's just a shy guy and he's actually very nice and one day I was in a more talkative, happier mood (instead of my usual quiet mood) and he was so nice to me, told me I looked pretty that day, asked how my day was going, etc. When people are nice to me, I get suspicious and question their motives and wonder if they're just being 'fake' and that they aren't *really* like that...they're just putting up a front to make themselves seem so nice and cool...Like when people ask to carry my things for me, or they laugh at my jokes, compliment me, reach out to hug me, I'm just like 'wow really? are they serious?' I always felt like I'm cursed and doomed in life...when life gets too tough I always get this thought telling me 'maybe you're supposed to be depressed and commit suicide, maybe you aren't meant to live, you're a failure at almost everything anyways'. I hate seeing other people happy... I think I'm too negative and I hate this...what do I do?