Well done nicnac! 
I totally understand what you mean about questioning why you are doing it to yourself. I even had the odd day early on where I've plopped on the floor and cried to dh that I couldn't do it and I wanted to give up. The thing is though I feel so much better about myself now, even simple things like standing up and realising I automatically stand straight and tall instead of slouching and its not a huge effort are really motivating me right now. Dh says he feels the same, we can actually feel our muscles working as we're moving around. Can't imagine ever going back to being a slob now I know what it feels like to get fit - its really reminded my why I was addicted to exercise in my teens and early twenties.
I invested in a new non-clasp sports bra yesterday as well - and some new trainers (which I'd promised myself when I got to the end of level 3). Sportsdirect have a really good sale on at the moment, got some tshirts for the boys for christmas as well. 
Don't worry about your tummy - leve 3 will deal with that. I've lost another inch off my waist and two inches off my lower abdomen doing level 3 and my lower abdomen stubbornly refused to budge throughout the lower levels. My weight didn't start moving until level 3 either - much like you, same couple of pounds up and down over and over.
Also don't worry about your diet today - one day off every now and again kickstarts your metabolism and stops it thinking you're going to starve - so enjoy it!
I find it really helps to log my food in My Fitness Pal, as I can see one bad day isn't a disaster, so it helps to keep things in perspective.
So, ever the masochist, I did BFBM warm up and circuit 1 today before I did level 3 of the shred. The good news is, I definitely found it less exhausting than yesterday, the bad news is, I still couldn't manage Natalie all the way through.
So that was day 17 of level 3 and my last day of my first round of the shred. I'm going to start again at level 1 tomorrow and force myself to do all Natalie, even if I can't keep up the reps. Strictly 10 days on each level this time and interspersed with learning the circuits for BFBM.