These latter two are not skills that most newbies master quickly. They are difficult and unfamiliar and carry the real potential for injury; the only way to learn them is to start small (typically from a kneeling position) and keep practicing until muscle memory takes over. The instructors usually have us work on them for a few minutes every class. It takes guts to even try them as a newcomer in a room full of more experienced students.
I've been taking these classes for about a year and a half now. I am far from graceful, and not even especially coordinated, but my skills are a lot better than they used to be. For example, I may not always come up on the correct foot, but if someone ever shoves me from behind while I'm walking, I'll be able to dive into a forward roll without getting hurt, and come back up ready to fight. I'll take it.
Taekwondo is a journey, not a destination. Everyone has physical limitations, and my best definitely is not as good as what some others can manage, even without the bum knee. But my yardstick is my own performance, not anyone else's, and fortunately, that's how the instructors view it as well. I'll stick with it as long as I keep getting better, even if only a tiny bit at a time--I was reminded this morning that baby steps really do add up.