Today, I've seen watched an interesting video on YouTube on aabout the history of these two pronunciations.

"I watched" sounds way nicer than "I have seen", plus if you are talking about videos/movies/TV shows, you should use "watch" instead of "see"

We all know the main differences between them, but I was really surprised to hlearn that British RP pronunciation isn't the classic English pronunciation.

It was just developed in the 19th century.

At Russian schools, students learn British English in most casesof the time.

Ands it turns out that, saying the 'r' sound has always been the part of the English language until 19 century once again.

I tried to make the 'r' sound myself, but I can't do it that well and I always end up twithsting my tongue being twisted.

To being honest, I like how British English sounds a bit more than American English, however, I have to admit that sometimes it's hard to understand.

For instance, I've been watching a TV series 'Good Omens', and it was easier to understand a few American actors on there rather than the main cast of British actors.

Today, I've seen watched an interesting video on YouTube on athe history of these two pronunciations.

"Have (verb)" is used for a past event that occurred before another event. Example: "I have watched a video on YouTube before writing this". "(Verb, past tense)" is used for an event that occurred sometime in the past. Example: "I watched a video on YouTube about history."

So it made more sense to edit out "have" from this sentence.

I changed "seen" to "watched" because "watched" implies that you noticed it for an amount of time/paid attention to it. To see usually has the meaning of casually noticing something with your eyes.

It was just developed in the 19th century.

"19th century" needs an article before it here.

Ands it turns out that, saying the 'r' sound has always been the part of the English language until 19 century once againthe 19th century.

You don't start a sentence with the word "and" in formal writing (but you can do that in casual writing and speaking). Also, "as it turns out" is a common transition phrase.

I tried to make the 'r' sound myself, but I can't do it that well and I always end up with my tongue being twisted.

Adding "myself" here is redundant since you already used "I" before it.

Also "I always end up tongue-tied" is another way to say "I always end up with my tongue being twisted". I would normally use "tongue-tied" in this situation as an American.

I'm used to leaveing it out as I was taught at school.

To being honest, I like how British English sounds a bit more, h. However, I have to admit that sometimes it's hard to understand.

This sentence looks better if it is two separate sentences, so I made it into two sentences.