Hello, Yuriy. This is really high quality writing. The only mistakes are generally in the 'finer points,' such as articles (they must be a nightmare for students) and some prepositions.

Hello everyone again! My streak was over/has ended, it's a pity but nevertheless I continue to write something about I think [you could, grammatically, say "something about which I think" but it would sound strange, better as "something about what I'm thinking"].
Today i want to speak about motivation and demotivation (my browser underlined the word 'demotivation' and I don't know if it is correct, I hope you understand me [-perfectly. The spellchecker probably expected a hyphen: de-motivation, but that isn't important]).
Let me speak about myself. I have been learning English about fifteen years. Can you image it? I learned it in my school and have got a good mark, but I really didn't know English, I learned it in my university for two years but if be precise to be precise / if I'm being honest, I only went to the lessons and did nothing, our teacher couldn't interest students, her lessons were very boring.

Then I had break for two years. When I finished my bachelor's course I had to pass an exam in English to be enrolled on a/the master's program. I passed it and forgot about English for two years again.
Look, now I'm twenty four and now I understand I want to know English, I want to speak in it. To be precise I seriously started learning one year ago, maybe a little bit more. Firstly I did it by myself then I went to English courses, I finished pre-intermediate level and now I learn/study (теперь?) English on a courses with a person from Pakistan, who knows English well and almost doesn't know Russian [maybe, "but doesn't really know any Russian"], besides (that) I use duolingo and a few programs. I've started to communicate with people who don't[множ. с 'people'] know Russian or Ukrainian and I understand (them)- it is cool when you can understand people from all over the world. It made me more persistent in learning.
Some time ago I've decided to write notes over here.
Maybe you have questions? What I want and why I write about motivation? Because I'm always looking for it but sometimes I can't find it.
- I don't need to use English at my work
- I haven't use English in my life I don't use English in my (daily) life (generally) 
- I was abroad only one time/once and I don't know when I'll be able to go abroad again one more time
These[множ.] factors demotivate me. What can you recommend for me? Maybe you overcame these barriers.

Thank you if you read my short note and thank you once more if you find any mistakes and tell me about them.
Have a nice day!

I hope you keep it up - however you're studying, I'd say it's working. For motivation, I think of all the things 'sealed' in the Russian language - whether that's stories, views of the universe or versions of events - that one day I might be able to understand. I try to bear in mind that uses for Russian might surprise me in the future, and that I can't imagine them all yet. It also helps me plan to travel, and so to save and hope (thought I can't travel for a while, probably ages).

I get demotivated by the same things. It's stupid, actually, but I even work with two 'russophones' (not russoph<em>obes</em>!) - one Russian, one Ukranian, but I never practise because we hardly ever meet, plus I have speech difficulties. I like to think of interests of mine, and how they will also exist in Russian-speaking places, but in a special, different way. And I need to learn the language to understand the difference. I am, of course, no good at giving practical advice!

Hi Yuriy,

I agree with Dominic, your writing is very clear and fluent.  I just thought of some expressions that sounds natural that you might like.  (I am American, by the way, so that may influence my choice of words).

Instead of:

I learned it in my school and have a good mark

I studied English in school and got good grades.  -- If you say "learned", it implies you learned everything about it, or mastered it.  In the U.S. we say "grades", I think in the UK they say "marks".

our teacher couldn't interest students

The teacher couldn't get the students interested.  OR

The teacher couldn't hold the students' interest.  OR

The teacher couldn't motivate the students to learn.

(In the 2nd sentence, interest is a noun, so you need the possessive apostrophe.)

Maybe you overcomed this barriers.

Maybe you have been able to overcome these barriers?

Maybe you have found ways to overcome these barriers?

It sounds like you are good at making your own motivatio\n! 

I used to study Russian as well, in highschool, but that was a long time ago.  I can still read pretty well, but I can't speak or write.  Maybe some day!!