Being Courteous Is Hard?

I'm not exactly Miss Sunshine, I'll tell you. I don't want to be anyhow. What I do want to change is my rudeness, for some odd reason I always end sounding rude. It happens when I write, when I speak, and probably even now. I'm guessing it's word choice but sometimes I really don't want to sound like a smart-@$$ or stuck-up or like a whiner. Sometimes I don't have any intention of sounding rude at all but it was comes off as I hate you, leave me alone. I don't really hate anyone. How are ways to sound nice and polite? I don't want to sound nice but just polite. I make enemies because of my statements that have no rude intention and also because apparently I give dirty looks. I don't even know that I do that. No wonder people always ask me if I'm mad at them. I'm not mad I just look at people like that. What are come advice, tips, etc.