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Mental fatigue from dieting can be pretty strong, and its easy to bounce back to old habits.
After a fast diet, working out what your 'maintenance' intake to hover around your desired weight is pretty important, because going back to your pre diet eating will likely get you back to pre diet weight.If you're trying to battle an upwards trend of weight, you have to make a fair shift on what you're eating on the daily after losing the weight, as your pre diet habits were to eat to surplus.
If you're at a steady weight previously, then its a bit easier to maintain your weight after a diet, as you had self regulated to your previous weight, just need to self regulate to a slightly lower weight.
It can be very easy, and take a long time, or it can be really horrible and take a few weeks.
The former is likely way more sustainable, as you are going to be eating pretty much the same amount of food post-diet, as you are during.
I'm aiming for 0.25 - 0.5kg a week to drop 10kg, by just eating more sensibly (no crisps, chocolate, biscuit binges), basic CICO, but without calorie counting.
Last year I dropped 10kg at 1 - 1.5kg a week, and it fell apart afterwards. The difficulty for me was not the hunger, but the mental effort needed to plan and manage a calorie restrictive diet, with very pedantic calorie counting, in such a way that I was eating "normal" food, and varying my diet a lot.
Horses for courses, though - what works for others may not suit you.