How do you know or test whether a game is too difficult or impossible? Guess, for example, that the creator of Flappy Bird could not play through his entire game and many others who create games are not as good at playing as those who just play.
How to find the boundary between difficult and impossible play?
Get the target audience/group to play test it, and listen to their feedback? Play testing it yourself is not the same. You know the ins and outs of the game, and will have tested certain game actions to check it worked.
For a player who hasn’t played your game yet, they need to go through that discovery phase. For you, that discovery phase is a hinderance and no fun. For them, it may be the best thing since sliced bread.