Bad things do happen to good people

I have not done anything majorly wrong in my life. I have (or at least I try very hard to) not hurt anyone but sometimes I say things that are totally not what I meant to say. Why can't I speak my thoughts? Someone once told me: if you don't have something nice to say don't say it. I really try. I try really hard. But my words seem not to match my thoughts or my intent. I really don't know what I can do anymore. I HATE how the world can easily misunderstand what I say. I really really hate it.