Sutra I.14: That practice becomes firmly grounded when well-attended to for a long time, without break, and in all earnestness.
This is the sutra most people skip on the way to the more interesting discussions of samadhi. It is, I would argue, the most practically important sutra in the collection.
Hypatia speaks of self-knowledge as requiring patience with what is uncomfortable to see. I speak of mastery as requiring patience with the pace of change. What they share is a resistance to the modern preference for rapid transformation.
Mastery is not a state you achieve. It is a direction you maintain. Notice: without break. The sutra does not say "with intensity." It says without break. The consistent moderate practice always outlasts the intense intermittent one. This is not motivational advice. It is a description of how the mind changes — slowly, through accumulated contact, layer by layer.
What do you want to be genuinely good at? Everything else follows from taking the answer seriously enough to return to it tomorrow.