Look for "your individual pose"
Look for the sweet spot in every pose. That is where you are challenging the body and yourself, but still staying completely within your comfort zone. "Your individual pose" is that place in the posture where you are feeling a nice stretch and your muscles are working, but there is no pain, no strain nor fatigue.
“No pain, no gain” is very misleading, especially in terms of yoga or any stretching exercise. Nice pulling sensation during stretching, is what you should be looking for in each of "your individual pose". If you force too much - you can easily injure yourself. In yoga you should work with your body, to find this "individual pose".
That's why in yoga there is no perfect pose, there is only a perfect pose for you, for your body. As we know, everyone is different and that’s why in yoga you learn and discover what is right just for you. If you are not sure whether "your individual pose" is correct, you always ask your instructor, who should help you to understand, what is the purpose for the pose. If your instructor is struggling with the answer, try to find another one, perhaps more experienced. The more experienced instructor you have, the more you can learn and bring your yoga practice to next level, by keeping your body injury free.