The difference between co-teaching a class and teaching a class by yourself is pretty vast. It’s the difference between running a relay race and running a marathon. (At least one presumes. I’ve never done either.)
I started my summer service design class with 20 (mostly grad) students. For many, it’s their first design class. There’s nothing quite like the terror of being the sole person to set the agenda, assign homework, lead the discussions, teach the lecture, and oversee exercises. Not to mention things like grading and taking attendance—the class is too small to have a TA or grader. Sure, I hear you saying, this is what college professors do, and you’re right. But actually doing them the first time alone, even after co-teaching the exact same class last semester, is intimidating. What if I do it wrong? What if the students hate the class? What if I forget something really important? What if I screw up the order of readings or exercises? It’s a little like your first solo drive in a car after getting your license. You’re just praying you don’t drive your parents’ car into a ditch.
I’m told it gets easier once you’ve taught a class a few times. Just gotta make it through the first ones.
One thing that I learned my first year on the tenure track: ask for feedback about a third of the way through. You could do this in a survey to get more general feedback -- learn what's going well, what's not so clear, what they liked to read, what they found challenging. And then you can discuss more generally what you learned and make changes where necessary. I found that it improved my final evaluations, but more than that, it let all of us talk together about the experience of the course. (Worth noting that it's scary to do if things aren't going well -- but it can help put them on the right track too.)