But I am finding it quite hard, as all other aspects of my life are "attached" to the Portuguese language. I wonder how people that have English as second (or third, fourth, etc.) think.
This question is usually useful when reading new docs or learning new concepts.
Can you watch movies with the voices in English? It helps to learn to understand. You can watch them first with subtitles in Portuguese, then subtitles in English, and then without subtitles. (I'm not sure if it's helpful for your objective.)
But I can't do elementary math in English. For something like 25+17 have to fall back to Spanish. (Most of the time I even read the numbers in Spanish.) (Perhaps I need to learn the addition and multiplication tables in English???)
(And about prepositions: I make a lot of errors. I'd probably be more accurate if I use a random generator to choice the propositions.)