When I was growing up, my mother frequently reminded me of an old proverb: "It's easier to ride the horse in the direction it's going."
Everybody's concentration wanes over time. We cannot expect to focus 100% on something for hours and hours on end every day. If we do have that capability, it is usually borne out of either deep interest or deep fear. It is rarely, however, sustainable.
Even when we have developed the ability to concentrate intensely for long periods of time, the effort is taxing and draining, and depletes our energy for other activities. Language learning requires a lot of mental space — so how do we use our powers of concentration to maximize our efficiency in learning?
Concentration waxes and wanes like the phases of the moon. When we start an activity, we begin with great enthusiasm and energy. But as we progress, we tire and our focus falters. That is completely normal. Rather than fighting this tendency, we can accept it and use the natural cycles of concentration to improve our language learning.
We can plan to spend short amounts of focused time on high-concentration / high-value activities, such as internalizing our language maps; as our concentration wanes, we can spend longer periods of time on exercises that are mentally easier. When we lose concentration entirely, we can still surround ourself with the language externally. Then we can return to high-intensity activities fresh and rejuvenated.
Many people punish themselves for not being able to spend hours a day at a desk concentrating vigorously. But that kind of intensity often becomes unproductive because it isn't sustainable. Instead, by learning to follow the natural cycles of our concentration, we can greatly increase the amount of time we spend working on a language, while simultaneously reducing the mental strain. This is not to say that we don't push ourselves to focus mentally — rather, we accept the cycles of concentration as sustainable for long-term language learning. We ride the horse in the direction it's going.