I have a bad habit that I definitely need to work on, t. That is, if I don't watch out, I'll always bend up snacking around and even pipigging out on food.

You don't need to separate the sentence but it makes it less comma heavy and emphasises that you don't need to memorise the whole structure of the last clause to understand the next one. (FYI though, if this was an exam they'd probably rag on you for verbatim copying the question as the start to the answer)
"If I don't watch out" is correct. Looking at the context I'm assuming it's informal, but do remember that "watch out" is seen as colloquial/informal by some.
"I'll always be snacking around" is technically fine but the emphasis feels like it should lie on you always resorting to snacking rather than the act itself. Then we can drop the continuous tense and also remove the "will".
"to pig out" is correct but to use it here you need a preposition, namely "I pig out on food" since pig out is intransitive.

I'm unsure if "even" means you're saying it's a rare occurrence (in which case you'd say "always end up snacking around, and sometimes I even pig out on food") or if it's just an addition to what's already said, which would then be "always end up snacking around and pigging out on food".

This is less of a grammar/language issue, just me talking really, but I do feel like there's some "context" missing from pigging out in this story. I'd typically expect to see "pigging out on food I find in the fridge" or "pigging out whenever I get takeaway" or some other context that separates it from snacking around.

So, what I do, or at least try to do, is say totell myself things like "You don't need it, you don't want it." and going to drinking water instead.

The comma isn't necessary after the "So" since you're following it with a dependent clause "what I do", which makes the entire thing one clause.
"to tell" is usually the verb we use when ordering ourselves, so it should be "is tell myself".
"go" requires the infinitive, so you'd say "going to drink water"

Following a diet has always been a challenge especiallyparticular challenge because of this fact.

The "always" is implied since you're using the present continuous and "has been". You can put the especially earlier in the sentence for more emphasis. Here, I replaced it with "a particular challenge" which means the same thing but you could easily say "has been especially challenging" instead. The benefit of "a particular challenge" is that it sounds better with the always.
You can also remove the "fact" at the end of you want. "Because of this" is fine here, and generally you don't ever need to qualify it unless you're clearing up confusion.

I love healthy food and I love to cook, but every now and then I catch myself falling on this againgoing back to it.

This could be fine depending on how you view it. If "on this" is talking about the issue as a habit then it's fine, although maybe "falling back to this" would be better. "falling for this" would work if you're talking about it as a kind of mental trick or deception, which you typically "fall for". I think the best would be "going back to this" though, as it's less likely to confuse. Maybe replace "this" with "it" since the habit is the topic of the text.

I have a bad habit that I definitely need to work on, t. That is, if I don't watch outcontrol myself, I'll always be snacking around and even pig out food.

So, what I do, or at least try to do, is say to myself things like: "You don't need it, you don't want it." and goingI instead go and drinking water instead.

I love healthy food and I love to cooking, but every now and then I catch myself falling inton this again.