There are a lot of common misconceptions about fossilization and language development. It's impossible to correct all of them in a post here, but I'll address a few that have been mentioned below.

Fossilization is a stage at which a second language speaker seems to cease making progress toward becoming more targetlike in his or her use of the language, so a "learning plateau" is a reasonable analogy. The comparison wherein "the L2 learner has his own linguistic system" that's still influenced by L1 and other things is known as the "interlanguage." The question researchers cannot conclusively answer is whether or not that "plateau" is reversible after a certain point, be it age, fluency level, etc., in order to start making progress again.

Different people are motivated by different things, which range from need (to pass a test, to get a job, to watch movies without subtitles, to make friends, etc.) to learning style (preferring to study from texts, liking/disliking impromptu, small-talk with people just to practice, preference for/against learning formal rules, and aptitude). It is easy to remember verb conjugations. There is no single formula.

Finding someone who can correct your errors tactfully and effectively most certainly does not need to be demotivating, depressing or draining whatsoever. I'm a very fluent non-native speaker of Spanish, and I actively request that my native speaker (NS) friends correct me when I make a mistake, or use a phrase that sounds funny in their dialect, etc. How else will I learn? It's fascinating to learn little details like that now after so many years. As long as they don't do it in a mocking or condescending way, or at a socially inappropriate time, why wouldn't I want their help? Of course, if you correct a beginner every time they try to utter a sentence, it could be discouraging - and obnoxious - but everything in moderation.

One of the biggest cognitive challenges is whether or not L2 speakers can learn to consciously notice differences between their L2 efforts and the way a NS talks. There are decades of research on this (my own included) and I'll gladly give references if anyone actually cares. Noticing falls into two broad categories. First, the ability to "notice the gap, " i.e. hear a NS say a sentence and think, "Hmm, I understand what he meant but I've never heard that word before; it must mean ___" or "I know what he meant but it would not have occurred to me to say it that way; I'll try to remember that for later." The second is the ability to "notice the hole, " i.e. when the L2 learner is trying to speak/ write and realizes that his "interlanguage" lacks a word, sound or structure needed to accurately convey his own thought. If he can seek the input necessary to fill the hole, he has a much stronger chance of acquiring it. The thought processes involved during that moment are holding the forms (or lack thereof) in working memory, and the longer it stays there for further processing, the greater the possibility that it is retained in long-term memory for later use. The NS interlocutor can help promote noticing through corrective feedback (also a subject of decades of research, for which I'm also happy to provide resources if anyone is genuinely interested.)

I have been using a variety of strategies for years as a language coach when working with my clients, whether helping IT executives from India and Egypt learn to write more grammatically accurate e-mail or helping priests from Nigeria improve prosodic aspects of their pronunciation (i.e. stress and intonation patterns.) Each person is different. I have found no evidence to support the argument that a person who has fossilized cannot begin to make progress again toward a more target-like L2 use at least in some areas, with the right motivation, input and effort. The question is only about how much progress, in what areas, in how much time, and through what methods.

Adapted form: https://www.quora.com/How-do-people-overcome-fossilization-andachieve-native-like-fluency-in-second-language-acquisition Acesso em 22/09/2023