Its mostly looks like your playing with the opacity. Its overlapping because your overlapping opacity. Opacity is how transparent your color is. But the more you add to opacity, the more you get less transparent.
Keep on layering, or try with full opacity instead.
When shading: First, turn on size-pressure link and opacity-pressure. Use a very, very low master opacity - on the top bar (I like about 5-8%, use lower for farther-apart colors and higher for closer together ones) and try to cover the whole area you want to change with each stroke (don't lift!!!) then a little less of that area where it's supposed to be changing repeat repeat repeat. After it's pretty close to what you want, use alt+tap to select inbetween colors and then smooth out. (Even before you do that, it usually looks ok though [link] )
Instead of trying to paint light by adding vertical strokes in the direction of the light, paint the lit surface of the sphere by making your brushstrokes follow its roundness. This way you can use the "grain" caused by the interacting brushstrokes to make the gradient as the light fades off down the sphere.
[link] See hpw the strokes follow the 3d shape of the sphere along the edge of the light, instead of plunging through it?
It looks like you are following a MGI Photosuite geek. With MGI Photosuite they don't have a soft edge brush like Gimp or Photoshop. They only have a hard edge.
I can see your problem.
You need to select an round area, fill it with color. Then use a soft edge brush that is made for blending. You use a brush the same size as the circle and make a bright spot where you want the light to hit the ball. Then half that size to shade around the edge, then half that and make the shadow area darker.
You never brush from the bright spot moving away from it. The bright spot is just a spot. You make the spot then go around that spot with less flow.