[aviaro]

I've been trying to become fluent for over 12 months, having been passively exposed to this language for over 9 years. I'd say that mastering English beyond intermediate level is extremely difficult.

You have to be constantly aware of what you are saying. Also, you need this kind of loopback that allows you to detect your mistakes and learn from them. This is a difficult skill which requires practicing.

You also need to have a social and communicative personality, so if you are introvert than you are less likely to master a language. I am focusing on speaking, because it is the most important aspect of any language.

As others said, when encoutering a new word you should check the pronunciation and commit it to your memory. This in turn allows you to practice your inner voice while reading. If you don't do that, then after some time you will reinforce your accent.