I say stuff that I don't mean, but I can't help it!!

Just tell them you don't mean to sound that way.

I had the same problem growing up, and it took a long time for me to really become aware of it to where I could control it, and it took even longer to be able to catch it and adjust. You are right, it's typically not what you are saying, it's that tone in your voice that adds to what you are saying, and makes it seem negative.

For some people, I know it's merely controlling their breath, because sometimes they get talking and they start talking out of their nose, and that adds to the problem.

For me it was just learn that tone I naturally hit, and controlling it, and working on it, until I changed it, so that I sounded differently.

Good luck!

You have to learn how to watch yourself. To figure out what kind of things bother people and then bite your tongue when you want to say something like that. I had a problem with the exact same thing when I was younger and it led to trouble with relationships, school, work, everything really. I've kind of learned that the thing I often think of first isn't what should be said and I've been mostly successful at suppressing it.

Tell them upfront when you are meeting someone and engaging them in conversation that you tend to do this. That way when it happens, people know already what the deal is. Also make sure to say that you are working on this problem.

Good luck!