Yoga has been called a "breath practice" by many experienced yogis. That's partially because many styles of yoga link breathing and moving. But it's also because the breath can reveal the truth about our condition, even when the external form of the body does not. The breath never lies!
When we practice yoga, our hope is to approach our edge in each pose, while keeping focused and centered, with the breath full and steady throughout. If you're ever unsure whether you're doing too much or not enough, listen to your breath. If you can't keep breathing evenly in a posture, it's a signal that you should back off to a simpler version of the asana—or take a rest.
This is also true of our daily lives. When you find yourself feeling tense, stressed, or overworked, try checking in with your breath. If it's even, calm, and rhythmic, then you're probably going to be OK. If it's choppy, short, and strained, then take a break, find a moment of relaxation, and wait for it to return to normal.