The transition from the rank of Brown Belt in Shotokan karate to Black Belt, represents more of a shift in the student's character and attitude than it does any marked increase in his or her technical aptitude and skill level.
Really, the earning of a Black Belt represents an initiation. A joining of an order. You earn the title because the sensei feels you are ready for it, not because you can execute all the moves flawlessly, though that is certainly part of the decision making process.
An intense amount of work was required to reach this point but work, you now see, that was preparation only for the initiation.
Now, having crossed the threshold, you are truly a beginner.
You start walking a much narrower path, where even the slightest misstep can send you falling down the mountain. With a narrower path, however, though the stakes may be higher, the way becomes much clearer than it was before. Decision making comes easier. You are less likely to be pulled astray from outside or inside influence.
Your energy is contained. In a way, your control is less, and your options seem limited. You are different. Your experience of the world is different and it will prove difficult, at first, to relate to others who do not share your discipline or level of commitment to know the truth.
This is a grinding phase of the journey where you constantly refine your understanding and skills, and where every imperfection of thought and rough surface of character is sanded out. Containment is therefore highly necessary. If you were able, as you were before, to pick and choose your every move, then this vital work would have little chance of succeeding and you would do disservice to all the seen and unseen elements investing in and supporting your cause.
Patience takes on new importance, as gains come more slowly and are harder to earn. Eventually, the desire to achieve more fades altogether, as you come to realise there is really no getting "there" but, rather, a deepening and joyful experience of being "here."