How can I make my yoga practice sustainable?

Don’t put in so much effort in the beginning that you cannot sustain. Practice as much as you feel like to. When you start enjoying it, you’ll naturally want to practice more. When you practice more, you’ll get better at it. When you get better at it, the same practice will feel less effortful. So with the same amount of effort, you’ll be able to practice more. Continuously in this cycle, you will keep getting better while putting in an amount of effort that you can comfortably sustain.

Doing too much is stressful. Doing nothing is boring. In the middle is where you can be happy and achieve progress too. When you are happy to do something, the habit will naturally sustain — no effort necessary.