I totally get what you are saying!! I am the same in that I know a lot about nutrition and health but I've been finding it difficult to exercise lately and stick to my healthy diet. I am also a huge believer in a no dairy, no sugar diet as it saved my skin and totally cleared up my acne. For the last couple of months I have "cheated" way too much and my body is making me pay for it in the form of nasty, nasty periods! (And the return of acne)
For me, part of the problem is simply the time of year. It's freezing cold out and i just want to hibernate in my bed or watch TV and EAT! It's been so hard to find the energy to do much!