Balancing the pacing of the game is hard and tricky. At the beginning of game design, I always thought that the good game is designed, by very intelligent people. It’ll just work well once published, if it is designed carefully.
Where to be hard, where to be easy, those should all be designed and it will just perfectly balanced. It turns out to be false, what I learnt thought the development is that the good game mechanics, pacing, and even balancing requires strong playtesting. You can never predict anything just by deign and play it yourself. At certain stage, it is mandatory that you bring some outside helps.
Those playacting are like putting a magnifier to your game, every annoying deign decision, imbalance, confusing just got nowhere to hide. It is best if every game can be tested over and over agin just to get things to the right spot. Where it’s not too hard to prevent the players from progress or too flat to make it interesting.