Namechange as this could be quite outing!
I'm Spanish (and Catalan) so was raised speaking two languages, then started going to afterschool English classes at about 8yo, 3hrs/week.
I was good at it, so I kept it up until I was 20, 3hrs/week every week during the school year. I also started an English degree at 18 in Spain. At 21 I did an Erasmus in the UK, 9 months, which solidified everything for me.
For me it was probably a slower process, as I was a child and sometimes took the piss, and did waste a lot of time during my late teens, but I am now completely fluent AND think in English even though I ended up marrying a Spaniard (I eventually moved to the UK permanently) with whom I speak in Spanish.
Sometimes I will struggle to come up with a word, but to be honest I struggle with that with either of my three languages, it feels like juggling a thesaurus sometimes.
I read in English almost exclusively (and I read a lot) and watch TV shows in English exclusively. I also consume other kinds of media mostly in English. I already did all of this before moving to the UK for my Erasmus though.
I have also worked as an English teacher when younger - I think it's really hard to learn a new language proficiently as an adult, especially if you don't have a solid grasp on grammar (not just English grammar, but grammar as a whole). I did find learning some Latin in Secondary School helped with that.