What does this statement mean. The "just", the "really hard"? Saying it almost let's one off the hook. "Well, I don't have to change, life is just really hard." "Well, I can blame another person or thing because life is just so hard." 

Is it really? Is it really so hard? 

Sure, of course at times life is rip-your-own-eyes-out difficult and requires every ounce of your will, strength, determinations, and full-body prayers and then it keeps squeezing; and on top of all that hard work, we are also required to surrender. But after a difficult experience or group of experiences, there is calm. This is the nature of it. 

How can we keep our focus on that specific truth? The law of pose and repose? The balancing principles of work and rest?

Perhaps, start by taking that phrase away and be more specific. Instead of "it's really hard," I, for example, could try to speak about the truth of the situation. I once heard Pamela Miles say that she hated the word "energy" because it lacked the depth of description that puts one in contact with the truth. 

Energy is really one of my least favorite words. Energy is like the New Age “whatever.” People just say “energy” when they don’t want to explore the exquisite reality that they are referring to.
— Pamela Miles author of Reiki: A Comprehensive Guide, from an interview with Elena Brower on teach.yoga

I could say ... 

I'm tired. I need to lay down on the floor and rest because I want to show my boys how to be when they grow up. And, if I don't give myself some space and rest and compassion soon, I am going to snap and then I'll be really tired from all the yelling and screaming and elevated blood pressure. So how about, I listen to myself and lay my body down for a moment or ten or perhaps just sit still and breath and feel what is happening.

This would be the truth of the situation. This is beneficial and specific. This can be refined so there aren't so many words and it could be clarified further. My point, with this example, is to consider getting clear instead of bypassing the difficult feelings and situation with a cliche. To be courageous and enter into a difficult situation with a mind, and soul, ready to receive the gift that a difficult experience has to give. This is not the easy way and sometimes we really don't have the energy to clarify in this way. But, this is the way of deeper understanding and a way to cultivate wisdom that is a gift to ourselves and can be gift to others.