Three months ago, I could do single leg box squats with some difficulty; at higher rep ranges, especially as the sets progressed, the squats got progressively more difficult.
In the subsequent training cycles, we worked more on the legs with many sets of bilateral squats, split squats, leg curls and glute bridge exercises.
Yesterday was a new training cycle and we went back to single leg box squats. The difference was remarkable. There was significantly more control to the squat, and the movement felt natural. In subsequent sets, I added on a 5kg plate and that too was quite doable (although, noticeably more difficult on balance).
The lesson is that no matter how hard a workout may feel, the body will adapt and develop requisite strength with time. The key is consistency and progressive overloading. With consistent and progressive overloading in a structured training plan, progress is inevitable.