I am a bad man and I want to change.

Let's begin at the beginning. There's a girl, call her K (I'm sorry, I'm paranoid). We've been going to high school together, now it's over and each of us went his/her own way. We weren't even friends; the thing is, I wanted her as a friend then and I still want it now. After over two months of not seeing each other, we decided to meet on the train back home. So there we are, talking, when I noticed that one particular lady took my seat, and slipped the info in our conversation. Later, being a type that doesn't like to make fuss, I chose another seat, sadly far away from K.
Later, we are back home, and she tells me that speaking about the lady was very rude and everyone heard it (I had no idea, I thought I was talking quietly), and she doesn't want to see me again until I change.
That wasn't the only occurrence; I frequently speak my mind, without thinking about the consequences. For instance: once, when I saw a postman, I said something like (to my friend): "Look, it's one of these guys that made me wait two weeks for [some random package]" (he heard it and we had a nice, quick discussion about delivery time).
So, please tell me, how do I become a different, normal man? How do I restrain what I say, not to offend anyone?

I don't see the problem? Understand that people can hear what you say?

I have a similar problem. I don't understand other people very well... (and it pisses off my boyfriend at times)