I'll weigh in as a novice adult learner... The lesson horse I've been on all summer has been c.h.a.l.l.e.n.g.i.n.g. Similar kind of stuff. Cuts corners. I learn to make him stop cutting corners, so he swings to the other side and starts trying to corner 4 millimeters from the rail. My knee is more than 4 millimeters wide, so this is Not OK with me. I learn to keep him off the rail. Wants to stop at the gate. Wants to run through his trot when he's facing the barn, then wants to slow to a crawl as soon as he's pointed away from the barn. Then he combines these. Races toward barn and cuts corner, then suckers on to the rail while slowing to a crawl. Drives me up the bloody wall...
...BUT I'm learning incredibly valuable skills the whole time. I'm learning how to handle these various situations - sometimes with more success than others - I never did manage to get a handle on the situation when he'd combine all of his tricks in one circuit around the arena. But I learned what I *should* do, and just as important, I learned what *doesn't* work. My teacher can tell me this stuff until she's blue in the face, but it's going to be impossible until I "get it" and once I've gotten it, it's there for good.
I'm also learning about myself - learning how to keep my patience, learning how to sort out - when problems arise - which part of the problem I created, and which part of the problem is being created by the horse. With this horse, 90% of it was him, and 10% of it was me (that was the teacher's assessment).
Way I see it, that's what horsemanship, or horsewomanship, is about. You build skills, but when push comes to shove, you're going to have to deal with what the horse hands out. A good horse-person is going to be able to take their 10% and get the horse to do what they want...anyway.
I could be riding an "easier" horse, but then I would know how to ride just that one horse, and I'd fall apart as soon as I rode a less easy horse. This way, it may not be "fun" all the way, but it's for sure "learning" all the way, and I'll be able to ride a much wider range of horses because of it.
I guess what I'm saying is, yes, fun is a part of this, but if you only wanted fun, you could just get trail rides instead of taking lessons. More important part, IMO, is the learning. And it might be helpful to your confidence if you focus on what you *are* learning, and what you *have* learned.