Hi Sally,
The best thing that I've done that has helped counteract those negative thoughts, it to think of something that directly contradicts them. If you are thinking "I'm such an idiot" think of GOOD things that you have done, or situations in which you were NOT an idiot - they ARE out there. Also, the more that you can recognize that these thoughts are in fact negative, the more likely that you are going to be able to see that they are unrealistic. Have you ever done Cognitive Behavioural Therapy? That works directly on recognizing those negative thoughts, and providing strategies to counter-act them.
Jacq