Any English learner who wants to get to a native speaker level can in theory do it by themselves. However as most people don't, it means that something else is needed. Elite performers in every area turn to coaches to ensure they are doing everything possible AND stop doing things that hold them back.
There is no reason why English language learners are any different. Turning to a skilled coach who understands not just English but the learning process is the surest way to learn a language to the highest level. The secret is to find someone who does that, not only gets you to work on your English but also works with you on the way you are learning.
The vast majority of teachers and coaches work on your English. There is a BIG difference between this and what I talked about in the previous paragraph. When they work on your English, this means your English may improve in the areas they work with you.
Whereas when a coach gets you to work on your English and works on the way you are learning, this means you are learning to improve your English and the way you learn. This is what needs to happen if you are to get to the highest levels. To get to these levels there are so many tiny changes and refinements that have to be made at every level that the only way you can do this is in your own time, as as you are using the language.
Over the years, I’ve met many people who desperately want to be successful but who are ultimately unwilling to invest in themselves. They think, "I can do this by myself. I can learn this on my own." Maybe they can, but in reality very few do.