I read a book on OCD - it's not my primary diagnosis but it does cause me a lot of disharmony - anyway, this book said to name these repetitive thoughts as something outside of yourself, like "the terrible monster," or something like that. It's easier to get rid of the thoughts if you say to yourself, " shut up you terrible monster, or trouble-maker, or whatever you want to call it, that's causing me so much distress, than "shut up these thoughts of mine that are driving me crazy." Put a name to them and when they enter your head, you can say something like "oh here comes.... again. It's an unwelcome visitor at your doorstep. I'm glad I remembered this, because I'm going to try it again - it really does help.