If your main incentive is progress, the lack of improvement can cause you to lose interest. Consequently, you may do less or no yoga until you close up and your body complains. Then you do yoga to feel better, and again you improve until once more you hit a plateau. The quality of mind that you bring to yoga is of utmost importance. In fact, most of the real limits that you confront in yoga live in the mind, not the body.
People think they are limited by their body's endurance - that tiring is purely physical.
I have found it is usually not the body that tires first, but rather, the mind which loses the stamina required for attention. When your mind tires, your attention wanes and begins to wander, and sensitivity to your body's messages diminishes. You treat the body with less care, and this tires it more quickly. I have found there are basically two personality types in yoga. The pushers are more into control and progress - the sensualists into surrender and relaxation.