There are at least four reasons why you have problem keeping your momentum throughout an entire piece. They may be contributing individually or together to various degrees.
1. You don’t know how to play the piece well enough.
Have you learnt the piece by heart? Do you know how the piece works, what each phrase, each note is doing, where it is going? Do you know when to change dynamics, tone colour? Are you conscious of how long each inhalation should be? How about the pause between ending an exhalation and beginning an inhalation? How are you ending each phrase? Why?
2. You do not have the strength and stamina to keep the momentum up for the whole piece.
Playing shakuhachi is a physical activity. Sometimes we simply tire before the piece is finished. This is related to #1. It is also related to the quantity and quality of your practice. Rule of thumb: One’s practice is never enough and also never high quality enough. High quality practice implies good energy levels, good concentration, alertness and awareness, in other words, good physical, mental and emotional health. It also implies stretching one’s limits, including those of your sense of hearing.
A good way to practice long pieces is to sometimes start your practice two-thirds into the piece. If you always start from the beginning and you tend to tire half way through, you may soon develop the habit of loosing the momentum before the end. This problem can be helped by sometimes practicing the end sections as if they were the beginning ones.
3. The piece does not resonate with you.
We all have our favourite pieces. Likewise, few if any of us like all honkyoku, or even all of the honkyoku that we have been taught. Don’t spend your precious and limited time playing pieces that don’t excite you. Some pieces need rest from playing too. The piece for you today may not be the same one tomorrow (people who play only one piece all their lives not withstanding).
4. The piece isn’t a very good one.
This may be subjective, and might be the same thing as #2. There are however, some pieces that I learned, and even more that I have heard, that I feel no obligation or inclination to play or to pass on.