How did you become fluent in speaking English ? I am in a dilemma!?

Hi

So I am in a dilemma. I was born in Spain and spent my childhood there, and one would assume that my mother tongue is Spanish. However at the age of 10 I moves to England. I became so fixated and concentrated in learning English that I neglected my Spanish. My Spanish is pretty bad. However the issue I am facing right now is that I am not happy with my English despite living 6 years in England. I feel like that I am not fluent. Whenever I speak English I often have to pause and use fillers because I have to think and find the suitable words, and sometimes I cant think of the word. I also sometimes struggle to express my feeling and thought that I have in my head on a piece of paper or orally as I struggle to make coherent sentences or I cant think of the accurate verbs and nouns. Sometimes I forget basic words like " rope " or "shoelaces"

I want to make English my first language. I have decided that I want to spend my whole life in the UK and altough I want to improve my Spanish but I want to be fluent in my English just like a native speaker is fluent in his language.

How did you become fluent in speaking English? Fleunt that if you are asked a question you dont have to pause or depend of fillers like " err" to think of the right word. Furthermore, to be able to express whatyou are thinking in your head on a piece of paper

How? Please tell if anyone has experience