See that picture? That's me, doing a side plank. With my knee down.
I'm modifying the pose for where my body was that particular day -- which, in this case, was not adequately warmed up because I was having pictures taken and not doing a full practice. And there's nothing wrong with that.
Yoga is meant to be adapted to your body, not the other way around. Which is why you should never be ashamed of making modifications. Nor should you be of using props, taking a break in child's pose, or stopping altogether.
No two bodies are the same; I'd like to think we all know that, but too often we compare ourselves to those around us. I'm guilty of this too, especially during YTT when I was in a room with a dozen other dedicated yogis, most of whom could go deeper into poses than I could or endure a more vigorous practice. It's hard NOT to look around and feel like you're not doing yoga "right" simply because you're doing it differently.
Our bodies also aren't the same every day. I may have done a modified side plank on the day that picture was taken, and then the next time I practiced I may have been able to hold a full side plank, or none at all. There are so many factors that contribute to how much energy, flexibility, strength, or stamina we have at a given time that we just have to go with it. Work to where you are at the exact moment you are practicing, not where you were yesterday or where you hope to be tomorrow. Listen to yourself, and modify as needed.
I'm going to let you know something: there is no right or wrong way to do yoga. As long as you aren't hurting yourself, do whatever works best for your body.