Assuming you're working in Photoshop, you should have a separate layer for the highlight. Duplicate it, apply a Gaussian Blur with a value of 2-5, and then choose a Blending Mode, something like Screen, Lighten, or Overlay. Scroll through the blending options until you find one you like.
And if you're thinking of submitting your art here, the general rule of thumb is "Don't." If it's good someone will eventually find it.
