Why is it so hard to do what’s good for us?

Confession time. As I write this, I’m going through a phase where I have only managed to get myself on my mat at home once a week. This is a hard thing to admit as a yoga teacher in training. But it is a common pattern for a lot of us – we are least likely to take proper care of ourselves when we most need to. So why is it so hard to actually do the things that are good for us?