It's not that I can't read on a computer screen for long periods. I just find it more enjoyable reading from actual paper and a Kindle is suppose to mimic that experience. My brightness is adjusted correctly and in some buildings with a lot of lighting overhead along with floor lamps there is always glare on some part of the screen no matter how I position it. I don't have the same problem with my matte screen PowerBook in those situations.I don't have any problem with reading for long periods on the iPad, and my job requires me to be reading on my computer for upwards of 10 hours a day. Try adjusting the brightness, or the lighting in the room -- the fact that the screen is glossy should make no difference whatsoever if the lighting in the room is correctly placed (or the computer is).
Might also be a sign that it's time for an eye exam.