why do people do this?

[Marund]

fall back into a bad habit, and totaaallyyyy give up or struggle to get back on again

is it like losing weight?
where if you start eating a LITTTLE bit of junk again you gain it all back

if you stop doing something good for a while
you have to work to get back to where you were more?

you fall way more behind than you thought ?

[FelineNostalgia]

Perhapss they didn't get to the root of the problem?

Like with dieting , people will diet to lose weight instead of having a good look at their behaviour and why they overeat and crave unhealthy food

[October]

I think that for many people it boils down to the "all or nothing" mentality, which is essentially a crippling approach because one will almost certainly fail and things will almost certainly go wrong at least sometimes. When imprisoned within the "all or nothing" mentality, many people will decide that a deviation from the planned course is a deadly omen (while it may merely be an opportunity to focus on something else for a while, without guilt), that they are incapable of following through their own plans, that it is not really worth it, and then fall behind, until the next (overly) ambitious "all or nothing" attempt, which will ultimately fail again, so the vicious cycle will the continue. The only way to successfully break the cycle is to carefully examine the underpinning mentality and modify it not to expect unrealistic perfection and not to consider some imperfections down the road to be fatal. It is also important not to overthink those imperfections, not to read into them too much (things such as: "I am incapable of doing this, I will never do it right, I screwed up today and this is the beginning of my end"), just acknowledge them and move on. Extend some grace to yourself, allowing yourself to be imperfect and not overthinking that bit of junk you ate, but also firmly continue to walk your road as though nothing bad happened and you may not even fall back into a bad habit in the first place. Falling back into bad habits typically happens when you give it all up and typically because you expected perfection to start with, all because of the underlying "all or nothing" mentality - at least it is so for me. I felt much relieved and was suddenly much better at accomplishing things I wanted to accomplish when I broke up with that mentality which has kept me imprisoned for many years, a slave to my own perfectionism, stunning my growth. For me it was really about realizing that occasional mistakes and deviations from a good habit I am trying to build will only be an issue if I make them an issue - but if I acknowledge them and move on without beating myself over that, not only will I manage to perserve on my way, but they will also become more rare.
I have still not grown out of that harmful "all or nothing" mentality completely, but I am working on it and I noticed great benefits in other habits formation as I work on this, because I am becoming more relaxed about it.