I really need help with my negativity?

Okay, so I'm extremely negative all of the time. I'm always rude and degrading (did i use that word right?!?) to everyone, even to those who don't deserve it. I'm always talking about someone and how they are a freak or ugly ... I always say **** (usually to their faces though, I am not two-faced) and I'm overall a huge complainer. I feel as if people think I'm rude, mean, annoying, and bitchy. I obviously know that I need to work on myself otherwise I wouldn't be typing this message. I've even challenged myself to not say anything rude, mean, or degrading to anyone for a whole day (needless to say) I only made it about 3 periods in school. Now that's just pitiful. Also when my friends start to hang out with other people, I think they are going to leave me and not be friends with me anymore, because of my negativity. Some people say it's insecurity within myself, but I'm not exactly insecure, yeah I'm a little over weight (but not too bad), and I have a few blemishes, but they don't exactly ruin my self esteem. It might sound like I'm in de-nile (however you spell that), but I would state if I was insecure, because obviously I'm reaching out for help. Also some say that I'm like this because of a past experience , but I really can't think of anything, but someone must have done something to me to make me create this wall around me. I'm so confused on why I'm like this, and I really want to fix this, but I just don't know how to. Please help me.