I always find it hard to get back to my normal productive routine after the long holidays.

We drop “the” when talking about holidays in general. “The long holidays” sounds like one specific holiday period, but here you mean holidays in general.

It's funny that in Russian, my mother tongue, the word "routine" only has a negative connotation, as if it were always were tedious and monotonous.

“Always were” is grammatically correct but less common and feels awkward.

For instance, staying up late and overeating is just as habitual for some people as it is to meditateing, write daiing diary entries, and reading for others.

Correct spelling of diary.
Change “it is to meditate, write…” into gerunds “meditating, writing, reading” for parallel structure.

The only difference is that the latter activities were entrenched deliberately entrenched, while the former ones were adopted unconsciously.

Verb positioning.
Correct spelling of unconsciously.
Add “were adopted” to make the sentence complete. Without it, “while the former ones unconsciously” is a fragment.

If you feel like you are spread thin, having a clear structure of your day might help immensely.

Spelling correction.

For instance, staying up late and overeating is just as habitual for some people as it is to meditate, write daiary entries, and read for others.

Attitude Towards The Routine

The topic isn't one specific routine; it's routines in general.

It's funny that in Russian, my mother tongue, the word "routine" only has a negative connotation, as if it were always were tedious and monotonous.

just a little more natural sounding to me--but that might be subjective

A lot of people believe in that, actually.

"believe in" has a more narrow application, for example, believing in God, fairies, true love, fate, etc.
"believe" (which can be replaced by "think") is what you should use here

However, I think that everybody inevitably has their own set of daily habits, and your attitude towards them greatly affects your life in the long term.

I'm not sure the word "inevitably" works well here, partly because I'm not sure that the statement is true.
It would be better not to switch from "everybody" to (you) "your attitude" and "your life." If you use "your" then use "you" at the beginning.
If you use "everybody, then stick with "their" which is gradually gaining acceptance even though technically it is a plural form.

For instance, staying up late and overeating isare just as habitual for some people as it is to meditateing, write daiing diary entries, and reading are for others.

The corrections are based on the concept of parallel structure in grammar. When the writer uses this kind of alignment, the reader can grasp ideas and comparisons more quickly or more easily, and parallel structure can also create more memorable sentences. It can be found in some famous speeches, for example.

The only difference is that the latter activities were entrenched deliberately, while the former ones unconsiously.

"entrenched" is not quite the right word here because it implies a kind of force that might be extreme. Yes, sometimes habits are instilled under duress, or by force, but I don't think you are in favor of that for the activities you list. I suggest that you need a word that shows these result from the choices a person makes and his or her self discipline. Once you find it, the sentence should be rewritten using a parallel structure.

It's easy to get stuck in a vicious cycle of poor habits and feel like your life is slipping away.

The juxtaposition of "vicious cycle" and "slipping away" creates an opposition of directions that is not cohesive. It kind of sounds like a washing machine doing a fast spin and then allowing the water to drain out. Try to match your images so they complement each other.

If you feel like you are spread thin, having a clear structure tof your day might help immensly.

The image of "spread thin" doesn't quite work. This phrase refers to having more to do than you can accomplish, which causes stress. You haven't discussed workload or responsibilities, so it feels like you jumped to a new topic that is unrelated to routine. However, the next sentence shows the connection you are trying to make. Perhaps "scattered" or "ineffectual" or the more neutral "not as efficient as you'd like to be" would be better here.

Since you would know exactly what and when to do, it canwould / could also act as a deterrent to procrastination.

This just feels like the right tense to me. Perhaps it is parallel structure...