Where does this guilt even come from?

Here's something I learned after the weekend of doom:

1) Eating shitty food after a week (weeks?) of healthy food makes me feel emotionally bad.
2) Feeling like I've undone a lot of my hard work makes me feel like I want to give up. Or rather, that trying so hard is not worth it when it's so easy and fast to fuck things up.

3) Shitty food after weeks of healthy food makes me feel physically bad.
    -my body feels sluggish and gross despite plenty of sleep
    -my body feels like it's...umm...trying to get rid of this food asap.
Ew.

So, I now am faced with conflicting feelings.  I have a strong compulsion to get in the gym and do cardio until tomorrow. I also have a strong compulsion to sit on my couch and feel sorry for myself. Sometimes a day off is good for the soul, or something. Of so I'm inclined to believe when I'm wallowing. (Edited to add: I went to the gym on my lunch break. I didn't want to. But, I thought that I would be more likely to do the couch-thing at the end of the day when I'm tired and feeling sorry for myself, so I just toughened up and went. 20 minutes going HARD on high resistance/incline on the elliptical, 10 minutes on the treadmill walking and running)

For the most part, I enjoy going to the gym. I love challenging myself to make healthy recipes that are delicious and filling. I am enthusiastic about being healthier. But I never realized that this whole process would come with this kind of guilt. Guilt over eating shitty food (and even more guilt that I enjoyed eating it). Guilt that I'm fooling myself into thinking this is a lifestyle change (how much of a change can be going on in me when I love eating fried chicken so much?). Guilt when I take two days off from working out (5 days a week is good amount of time to spend in the gym, but those two days off feel...bad).

Oh, and I also feel guilt/shame when people on the biggest loser still look fat and weigh less than me.

Obviously, this guilt is not from an external source. And I don't know if I should be trying to get rid of it, or if it's a good to have it.