When I hear things like:
"Oh I tried that DIET and then Christmas came and I lost all willpower and gave up"
" I had no will power tonight to stay away from those cookies"
" I couldn't muster the will power to make it to the gym tonight"
I get bothered.
I feel like when the body sends a signal such as "stay home from the gym", or "have a big plate of christmas dinner" it is telling you that for a reason. Perhaps your body needs rest, or needs a good meal because you haven't been eating properly lately.
Perhaps there is a physical/chemical/emotional problem going on the body that is asking to be taken care of. Perhaps it's an issue a chiropractor, homeopath, naturopath, or accupuncturist could help take care of? Especially if these types of instances are happening frequently. (Because sure every once in awhile we all don't feel like going to the gym or resisting the cookies). The most common reasons why I tend to make decisions that aren't totally aligned with my health goals are: feeling overwhelmed/tired, bored, procrastinating, not letting my emotions out, and feeling sorry for myself. Therefore by dealing with these issues I don't have to rely on willpower anymore!!
Anything that you continually have to force yourself to do long term is not sustainable, and therefore by making yourself do it, and sometimes caving and not following through you fall into a guilt/shame cycle.
You say you HAVE to go to the gym to lose weight, so you force yourself to go 5 times a week, if you end up skipping a week you feel like you lost your willpower, feel guilty, and feel as though you've thrown your goal out the window. (Maybe not always to the extreme).
Get to the bottom of why you have to use so much 'willpower'. Are you struggling with over eating, and need some counselling/support, have you not found a gym/trainer that motivates you and inspires you to come back to the gym?
If you skip a workout, or gain 5 pounds over christmas, or give in to a few cookies here and there- don't blame it on your will power. Make the conscious decision to skip, or give in, and be satisfied that you made the right decision for your body at the time. GUILT is making you feel like a failure as a person and only leads to further destructive patterns down the road!
This is something I have overcame over the past 3 years- if you are looking for support/guidance, please e-mail me firstname.lastname@example.org