It appears to me that you're using software to increase the exposure (brightness) to get rid of darkness and grey color of the background. But that's eliminating your shadows, which are needed to give your items "substance".
I think there are two solutions. The first one is to learn to use the EV button on your camera so that your photos are taken with the correct exposure and all you have to do in software is brighten and sharpen. And the second one is to learn how to use white balance. Your camera likely has a setting for it.
Your pictures aren't too bright, per se, they just lack depth. You don't want harsh shadows, but you do need some.
Getting a good exposure when you're taking photos on a white background is tricky because the camera is fooled by all the white. I explain it here:
http://thebluebottletree.com/photography-tip-why-are-my-pictures-so-dark/