How can people learn 7 languages when I can't learn even one properly?

I've been learning English for years. I had English classes already in the kindergarten. I use internet in English and read articles in English daily, yet I still find reading in English tiring. When I read longer complex sentences with words like "its", "it" "they", "their" etc. I sometimes struggle to find what they are attached to if there are multiple possible subjects. I need to reread some paragraphs to properly attach actions and descriptions to the correct subjects - sometimes I understand every word in a sentence but the way I read it doesn't make sense. I needed to invest a lot of time and energy to be able to read articles in English relatively fast and comfortable and there is still a big room for improvement.

The thing that concerns me, there are a lot of people on the internet bragging that they know 7 or even more languages. How can they learn 7 when I can't learn one properly?

If I had started learning German my level of English would be much worse than it's now and I wouldn't be good at German either as learning a new language from the scratch is very difficult. If I had started learning Japanese after achieving B1 in German it would have slowed the further progress of my German and English even more.

So how is it possible that some people know 7 or even more languages then?