I am just about to enter my 12th month, so I have had some of those days which you speak of (and will undoubtedly have more).
And you know what? It's OKAY, and it will continue to be okay!
The pre-Spark me would have let that derail me (which you also referenced). The difference now vs before Spark is that I don't let it do that anymore! I just don't! It really isn't productive and life is going to have moments / days, etc. where you want to go off plan or indulge. It's a moment or a day here and there - not tons of moments or tons of days all the time.
In order for this to be a lifestyle and not a "diet" (something you are "on" or "off"), you need to figure out how to shrug your shoulders and say, "Oh well. I enjoyed that and that was fun and now I'm back to more healthy eating." What's funny is that you will start preferring healthy stuff after a while!
I had a deep fried goody at the San Diego fair last weekend. I planned for it as best I could, and by golly, I was NOT going to feel guilty! They were out of the deep fried S'mores so I opted for the deep-fried avocado (came w/ dipping oil, no less - cause you need to add MORE oil to this thing! ha!). I ate the whole thing. It was okay but not the nirvana I had expected. The odd thing: on the drive home, I peeled and ate a fresh orange which was even more delicious than the fried avocado! Go figure! Guess it's true your tastes really do start to change.
This giving yourself permission to live a normal life is ultra liberating and because of that, I will NEVER be on a "diet" ever again! Yippieee!!! Will I be mindful? Yes; but I'm not driving myself crazy about it and I have continued to shrink in sizes (I don't weigh myself).. And because I know I can 'have permission,' I no longer binge like an inmate ordering his last meal. Hallelujah!
I can't tell you how grateful I am for this sane approach.
So relax; you're going to be JUST FINE, I promise. Deep breath. Put on a smile and just soldier on. it really can be as simple as that. You can do it!!