Ok, Quick tip. When trying to make things difficult, you never (ok, not always, but 99/100 times) want to mess with the players input, and when you do, you need to either relay that information visually, additionally, or via words. A good example is with sonic in the genesis games. He can go really fast and jump on a dime, but its can be difficult to both slow down and avoid enemies. Thus the levels in those games are designed to use that difficulty as its main challenge.  for your game, it would have easily been able to use something like a special floor type to make things slippery, or events that make the game's gravity stronger. Obviously, this is something that takes time, and I also need to work on, but I hope that you got something out of this experience!