First - congrats on actually achieving such an amazing weight-loss, you must've worked incredibly hard and you should be proud of yourself.
Second - I must agree with previous posters that you should seek out a qualified therapist to deal with this. Sounds like you still view yourself as that overweight person you once were, and even when you were at your highest weight, being disgusted with yourself for gaining .5 lb or feeling such overwhelming guilt is not healthy, good or constructive and will only lead to more anxiety, frustration and self-loathing.
Best of luck to you & take it easy on yourself!!