Things you can always change are how you think, feel and react to things.
Some of these types of books say that yourself is the only thing which you can truly change because, they say, everything else is an illusion or simply that trying to change others is a futile task. How you perceive reality is everything.
I find this idea to be very liberating and pragmatic and when I remember to utilize this in practice, things flow much better for me. I become more flexible.
As Bruce Lee says - "You must be shapeless, formless, like water. When you pour water in a cup, it becomes the cup. When you pour water in a bottle, it becomes the bottle. When you pour water in a teapot, it becomes the teapot. Water can drip and it can crash. Become like water my friend."
What you can change is a drop in an immense ocean. And that's ok. Yo can either ignore the rest of the ocean (denial), or make peace with the fact that the tiny drop of the ocean you can control is all that really matters.
Well... it does have the advantage of being true. I'm just explaining one positive method of coping with this vast and amazing universe. Do you have another?