I don't speak British English, so I could be wrong - but "I decided to educate myself on British English" sounds unnatural. It's very elevated language. Also, it sounds like you're saying you learned *about* British English, not that you learned the language itself. I would say "I decided to learn British English" or "I decided to teach myself British English."

I agree with Sara’s suggestions but would also add that the rest of the sentence sounds a little odd and is probably untrue. You could just say that you preferred the British accent, or thought that it was more useful to you, or thought that it would be easier for you to learn (and give the reason) Also you are repeating “decided”. In 2014 I decided to teach myself English and chose to focus on British English because .... (the American accent seemed too difficult, ..)

I would use the verb teach - I taught myself British English. Educated in this context sounds wrong - although not officially incorrect.

Sure, no problem. The suggestions below add more information to it, but your expression is fine. Your use in the question "in British English" is better than your example "on British English", but I think you could argue for using either "in" or "on". ...because I was not able to absorb the English <-- past "am" --> "was" tense maybe is even better here too, though I guess you mean it is still true.

"Back in 2014, I made the decision to learn English on my own and I decided to teach myself/learn British English because I couldn't understand/absorb the English from the USA, especially the accent."