Why O Why

Why do I do so well for a period of time then fall off the wagon? I can do great for a week or two, exercise every day not give in to temptations, then out of the blue it all goes in the shiter. I think I have to set a goal. I usually do good if I am working towards something. Back in August I was getting ready for a friends wedding & I did great for 3 weeks. A couple weeks ago I was getting ready for the race & I did great, then I took 2 days off from exercise & have not been able to get back. I have also been eating everything in sight. I am going to set a goal to exercise 6 days a week & control my eating until Christmas. I will then set a new goal after Christmas. I know it will be hard to control my eating/drinking with all the parties coming up but I am going to do it.