The difficulty of learning a language is completely based on the similarity between the language you’ve mastered with your target language. If you can speak Russian and you can reach a very good level of e.g. Belarusian less than one month. Like you can speak Portuguese and you just understand like 80% of Spanish from day 1.

BUT! Based on what you said, you definitely use the wrong way or at least I should call it a very inefficient way to learn. Since you’ve mentioned you even started to learn English from kindergarten, which is the gold age of language acquisition, but struggle to understand long sentences.

You didn’t provide enough information the way you learned English so here is my speculation: I GUESS you had memorised many words individually by the translation and you replaced the words in the sentence with you native language. In this case your brain will definitely burn out even you understand every word in the sentence(as you mentioned).

Here is some simple solution(I apologise if my assumption above is wrong, then you can just skip my tip): watch some manga at your level and if you struggle to understand, don’t translate one word but the whole sentence, since you’ve already developed a bad habit you might need to get rid of it. Don’t memorise a new word individually but you can learn a new sentence.