Is there something wrong with me?

It occurs to me that for a better portion of my 17 years of life, my personality seems a tad warped, I'm generally polite to strangers, I may tease a bit and all that friendly stuff. But to the people I care most about I tend to be a complete *****, pointing out their faults, being uncooperative, generally being mean, why do I do this? I want to be nice to the person but I cant stop myself from blurting out the meanest remarks or the things they really don't want to hear at the time. How can I stop myself from doing this? It's really starting to hurt my relationships, at least, that's what I think considering my boyfriend is getting annoyed with it and I'm sure its wearing on my best friend's nerves. It even bothers me if I listen to what exactly I'm saying.