There is a popular fitness quote by Fred DeVito that applies perfectly to the studying trap you mentioned: "If it doesn't challenge you, it doesn't change you."

When learning a language, if you only ever drill the vocabulary and concepts you’ve already mastered, you’re essentially lifting the same 5-pound mental weight every day. Doing the easy stuff feels incredibly productive because you get all the answers right, but it's an illusion. It gives you a false sense of accomplishment while keeping you completely stagnant.

To actually get BETTER, you have to force yourself to stumble through the difficult, unfamiliar material.

However, there is a hard limit to how much of that difficult work you can actually handle.

You cannot stay deeply focused on the truly frustrating, challenging parts of learning for hours at a time. Your cognitive battery drops fast when you're doing heavy lifting. If you try to force it, your focus will scatter, you won't retain the information, and you'll dread studying the next day.

To stay consistent long-term, you have to know your limits and use a timer. Give yourself a strict 25 or 30 minutes of deep, intense focus to tackle the hardest material. Once that timer rings, you are completely done with the hard stuff for the day. If you still want to interact with the language after the timer goes off, switch over to something passive, like watching a movie, so you end the session feeling successful rather than exhausted.

To be consistent, you need to be in a flow state.

However, you cannot stay in this state for too long.

The task at hand should be challenging enough to keep you engaged, but not so difficult that it causes anxiety. At the same time, it should not be too easy, as this can lead to boredom.

To enter the flow state - to have peak focus