Fighting irrational thoughts never works for me. I find it's much more helpful to distract myself. When I used to get really bad OCD type thought patterns, my therapist told me to schedule a specific time of day during which I was allowed to think these thoughts as much as I wanted, but only for that predetermined time period (for example, at 4:00 PM I'm allowed to indulge in these thoughts for 30 minutes). When the thoughts pop up before their time, you have to put them off until later. Often when later arrives, you find you don't need to think about them anymore. If you miss your 30 minute window, re-schedule it for the next day