Fading line using gradient tool in Photoshop CS6

I have a vertical line to fade from both the sides.

My try:

I have followed these steps:

Step 1. Own layer for line.

Step 2. Added Layer Mask to the layer.

Step 3. Selected the gradient tool.

Step 4. Clicked and dragged the gradient on the layer mask.

Problem: If I drag the gradient tool on the layer in top part of the line it's done. But when I move to the bottom side the top part is getting deselected.

How to overcome with this?

Thanks