Welcome from another Sydney sider. This site offers loads of inspiration.
Your 'essay' argues exactly why you should stick to this. DON'T cave in, don't treat it as another diet on the diet rollercoaster you have been on for decades. I don't know how to encourage you not to cave, but maybe if you do cave, treat that one (ONE!!) incident as part of your 20% and keep going. Plenty of people here to support you...good luck.