Great first game!
Pretty good game, but you should make it easier. I made it to level 8 on the medium stages, and I would expect to at least be able to make it to the hard stages.
Also you should think about adding some power-ups, for example something that turns off the fields, makes you invulnerable to red balls, prevents your progress bar from decreasing, etc for a certain amount of time. Those could come from time to time instead of a ball or you can make a point system, so you could buy these in-between levels (or just make some levels where you start with some powerups you can activate during the level)