Why I have a problem with English [or "Why I have a problem learning English].

I have my own a theory about why Russian people, who are learneding English has have problems with it. I think, that, the human brain of human is a complex system that is hard to fathom/understand. so difficult sistem and we couldn't understand it (полностью). When we trying to learn something new and don't finish it at once, twice, again and again. (Рано или поздно) our brains formed some kind of "barrier". in our mind. When we try to learned it again we encountered that with it barrier. It is [or "it's"] like when we start reading a book again and again never finished it. It becames tiring/unintersting not interesting and each time we try again it becomes more difficult to start reading again.

Sorry for my bad English. Help me, please.

Cheer up! In common folk tongue, it's what they call "Challenges/Obstacles". Without them, learning or taking up something new would not be as interesting or worth your while once you achieved it. Achieving your goals whether is to read a new book or learning a new language can be rewarding in so many ways. It becomes satifying becuase of that "Barriers" you encounter. This is the minds' way of saying "Hey this is something new?!". A best example would be... physical exercises are tiring but if you repeatedly do it (overcome the challenges/barriers) you'll get great results: Sexy, gorgeous and best of all healthy bodies. My best advice, try to find a way that you'll enjoy learning English. This goes with other activities that you might find difficult to achieve. Good luck!

Suggested correction  (Although this is not straightforward.  There are certainly other ways to express these thoughts.)

Why I have problems with English

I have my own theory about why Russians have problems learning English. (('I think, that brain of human is so difficult sistem and we couldn't understand it (полностью)':  Tough sentence to correct.  Could you try saying this in a different way to help make the meaning a bit clearer and easier to correct?)) We try to learn certain things and don't pick them up at once, or even the second or third time around.  Sooner or later, a barrier forms in our minds. Thus, when we try again, we encounter the barrier. It's like when we start reading a book over and over, but never finish it. The book becomes less and less interesting and ever more difficult to finish. 

Why I have problem with English

I have my own theory about why Russians, who learned English has problem with it. I think, that brain of human is so difficult sistem and we couldn't understand it (полностью). When we trying to learn something and don't finish it at once, twice, again and again. (Рано или поздно) our brains formed some "barrier" in our mind. When we try learned it again we encountered with it barrier. It like when we start reading book again and again never finished it. It became not interesting and each time it becomes more difficult to start reading again.

Sorry for my bad English. Help me, please.