I’ve learned the hard way that sadhana is just something I have to do if I want to achieve my personal goals. It’s really not more complicated than that. That said, the passion I have for my sadhana waxes and wanes. After a recent wane that lasted weeks(!) and a few dry mornings where I made it to the pillow, but, whatever, I took a look back to what I was doing when my sadhana was hot. There was a pretty specific asana / pranayama / mantra / meditation practice happening and things, meaning my life and demeanor, were changing for the better.
The biggest challenge is consistency. I heard in my head during an asana practice recently “If you will just go to bed at 9:00 each night you’ll have everything you want.” I know exactly what she means.