After weeks of being unable to do this, I finally found how to get shot of these horrible dragonriding games and move on to the proper stories.
For the first three quests (gliding, diving momentum). I got on the dragon and aimed at any ring I could see from the parapet. If I passed through it, I made the dragon jump to go upwards and tried to aim for another ring if I saw one. If I missed, I let the dragon fall to the ground. The secret I found at this point is not to give up the quest or use the timer thing, because using that sets it all to zero again. Not using the timer meant I kept the score for the rings I had managed to pass. Once the blue ovals on the screen filled up again, I made the dragon jump again and tried to get near a ring. Each time I passed through a new ring, it added to the score and it didn't matter in what order or direction I passed through nor how many times I tried. Once I had all the rings, I tried jumping up to the landing target until I managed that. Climbing some rocks to get higher before jumping helped me to reach the rings and the landing target. I'm not sure whether it matters, but I made sure I stayed inside the blue quest area marked on the minimap until I had finished the quest I was on. I needed very many attempts, but finally got through all the rings for all three of these quests.
For the final quest, ignore the guy who tries to force you to race him to the observatory. Just make your way to the bottom of the observatory in your own time by any route you want. There is a "ferry" there, which will take you to the top. Once you are at the top, the quest is marked as completed.