Is the cobra stretch so hard for you, too?

I have questions about a few other exercises, but I'll start with the cobra stretch: for me it's not even a stretch, but an upper body workout. It usually comes up at the end of workouts, in which I already tired my body out, so when I do it, my arms are shaking, and I feel my lower back contracting. Kelli and Daniel also say that your hips should stay on the floor, but I can barely keep my thighs on the floor during a cobra stretch. I don't think I feel my core stretching at all, either, and when they say things like "relax into it", I just laugh bitterly.

I've been working on my flexibility over the last few months, and I can see some progress happening with other stretches, but not the cobra.

Do you think my form is wrong, I need to wait for my flexibility to improve further, or that I'm just that out of shape? All three are valid theories! For now I'll keep doing it and treat it as an exercise that helps me enjoy the child's pose even more, since it's usually right after the cobra :-)