In my opinion, sometimes I become undisciplined because I don’t want something badly enough—I’m not desperate for it. That’s why I lose discipline.
For example, some time ago I became overweight and my health started getting affected. At that point, I began exercising regularly and became disciplined, mainly because I strongly wanted to improve my health. However, now that I have returned to a normal shape, I have again become undisciplined and stopped exercising.
So, I believe that when you truly and intensely want something, you naturally become more disciplined in pursuing it.