Yep, I very much agree with the one day at a time approach.
I would also say that it might even be worth writing off the 18pp you used on the chocolate so that you have more leeway with your weeklies for the rest of your week. You obviously can't do it regularly but the important thing seems to be getting you back into the right mindset with things and struggling on for the rest of the week in quite a restrictive way might not be the best approach to do this.
Big hugs and good luck. You CAN do this, and you WILL. xx