Hi. I think you have a really bad habit! I would tell several of your best friends/family you can "trust" and ask them to ask you "Is that true?" or something else you agree on after everything you say so you can learn to "correct" yourself, using their help at first. I use to put myself down but now actively argue with myself if I do :-) You need your "other" self in you that helps you learn to say things straight.

My middle name was my grandmother's maiden name, Danaher, and when I was your age I use to call myself Damn-it-her instead but then that got to be a habit and I realized it wasn't very funny but I had a heck of a time stopping doing it! It's like when people do a lot of swearing and then decided, after they have children or something, to stop and have trouble doing so. You have trouble telling things straight. Tell others and get them to help you! If they're real friends they'll help and not tease you too bad I bet :-)