
When I first started to do yoga, using CDs that I purchased on Amazon.com, I remember seeing headstand and thinking, I will NEVER be able to do that. And then I kept going, and I went to yoga classes, and I heard/read that if I kept practicing, any yoga pose was possible. And within a year, I was doing
Fast forward 15 years and I still cannot do Utthita Parsva Hastasana (among a myriad of other poses) with a straight leg, like this:

Is it because I haven’t tried? Because I’m lazy? I didn’t practice enough? Nope. It’s anatomy. Think about all of the parts of the body involved in this pose. In terms of musculature, we have all of this:

And, that doesn’t even include the bone and joint activity involved in the hip! Yoga poses often include stretching of muscles, but they also include compression. Stretching can be gradually and slowly impacted with regular practice, but compression (or bone on bone movement) will not change without serious injury. This article, by Esther Ekhart, does a good job of discussing this difference – https://www.ekhartyoga.com/articles/anatomy/tension-versus-compression-in-yoga.
While regular practice of this pose gets me closer, I have generally accepted that my body is probably not going to do this without creating a whole new set of problems. The literal shape of my femur bone may be what makes this pose so challenging and that I cannot change. Check out some of the bone images from Paul Grilley at http://paulgrilley.com/bone-photos/ and you’ll see what I mean.
Similarly, you’ve probably seen some people who can easily fold forward and touch the floor and others who just cannot. Many things are involved there, but one of them is the literal length of the bones in arms and legs. You cannot change that. It’s just how your body is made.
If you are in class with a good yoga teacher, you will hear things like “don’t push past your limits,” and “your pose doesn’t have to look like someone else’s.” Your instructor might adjust you for safety or alignment, but it will be in small amounts and the teacher will never push you into a pose. If your teacher is insisting that everyone should look the same and that anyone can do X, that might not be the right teacher for you. If it’s not the teacher, but your brain, it’s time to let that go. If a pose feels stretchy in a good way, go for it. If it hurts, stop. Don’t force your body into pain. That’s not yoga, it’s just
It’s your practice. It’s yours and yours alone. Don’t do anyone else’s or try to be anyone else. You don’t need a “yoga body.” You need the willingness to step onto a mat, come into yourself, and focus on the union of breath, body, and mind, using asana to help you with that. And that’s all you need.
Namaste!