Was really hard the only way I did get to the end level was by going through there with the main menus. I think I heart every level would be too much. Maybe like every 3 or 5 levels. And maybe make an uncommon drop so enemies drop a healing item like a wrench so you can heal on the long level. Also, a bit of the difficulty may come from the bullets being a little hard to see sometimes.
Thanks for the feedback, I meant increase the starting hp on all levels by 1. A healing item is a good idea, as that will keep levels tense since you can die before you get to the part with the healing. I might experiment with a light source on bullets, to see if that helps, or if that is too disruptive maybe colored pixels in the middle of the sprite. I am glad you enjoyed it!