When I was a kid, I reached an age where I could no longer do pullups. I found this very depressing. Determined to be able to do pullups, I starting practicing every day, hanging from a tree branch in my backyard and pulling as hard as I could. After some weeks, I managed one rep. It was a major victory. I kept at it, and my numbers increased quickly until I was able to do more than 20 pullups.
Since then, I have always been able to do pullups. In grad school, I took things to the next level by training with a weighted vest and doing several sets throughout the day. Since then, I have always been able to do at least 10 - 15 reps, even after periods of more than a year without doing a single pullup.
What does this have to do with language learning? The experience is the same with training. If you put the time in to do it right, and allow your new strength to soak into your bones, a certain amount doesn't leave you. Provided you've put enough work in, you actually change your composition, and you take ownership of the strength. If you do hundreds of reps of lightweight lat pulldowns, you'll never be able to do pullups. The same is true with language learning.
If you take the time and put in the effort to do it right, what you learn, you keep. Strides in language learning are best made through periods of intensive training, then solidified through consistent practice. The intensity can be either within a given day (e.g. 15 - 20 minutes of concentrated study), or over a specific period (two weeks intensive studying), or both. Reinforcing these gains is key to solidifying them and making them yours, but the language strength can be quickly gained through this kind of high intensity interval training.
I know many people who have studied a language for years, but fail to know any aspect of it well. This kind of lightweight, low-intensity training produces only small gains that are easily lost. Sitting down and really learning one aspect of the language is better than sorta kinda learning someâ€¦ Piece by piece, you'll get the whole thing down eventually, and if you do it right, it doesn't leave you.