Below is a true scenario described by an acquaintance who has been studying English for ten years now and her English is still at the level it was after the first six months. The scenario stops after the first six months.
You are a learner of English who really wants to improve your English so you take an English course at the best language center in your area. The course lasts three months and you study 3 days/week, each day for two hours. You don’t speak any English at the beginning but after the first three months your English is pretty good considering that you have been studying for just three months. Does it sound familiar for you?
Now, you can see a big improvement in your English skills and you decide to continue studying the next amazing course offered by the same great language center. The first course was a course designed for beginners but this one is a little bit more difficult. More than that, now you realized that the first course took a lot of your free time and other things are behind their normal schedule. You also know that for this little more advance level course you have to spend at least the same amount of time as you spent for the previous one or even more time. However, you decide to go on. How difficult can it be?
You started the course and suddenly realized that the vocabulary used in this course is quite difficult to remember. In the first course, some of the words were familiar, you could remember them from the 5th grade English class, but the vocabulary used in this second course is really difficult. More than that you have to remember the vocabulary used in the first course too. Anyway, you go ahead with this course and after another three months you complete it. But, surprise! You just realized that your progress done in this second course is just a fraction of the progress of the first course and some of the words and grammar points studied in the first course are now forgotten. So, you start asking yourself why this happened and what should I do next.
First of all, learners of English have to constantly review the vocabulary/language they have learned. You study more courses/lessons, you have to remember more vocabulary and grammar rules. Practicing your skills becomes more difficult and more time consuming. At that point, when everything becomes difficult you have to stop taking another course. You have to practice what you have studied until everything is simple, until you remember all vocabulary words, until you understand all grammar points etc. Read and re-study at home what you have studied until you dream everything in the night. Only then you can step up and take another course.