Then again, again again again and again!

Now, save copy of document (always good idea), then merge all layers (Ctrl-Shift-E, or Layer menu - Merge visible), then sqitch to Quick mask mode (Q), select Gradient tool (G or Shift-G), set mode to Reflected, and make the gradient by clicking mouse in the center and dragging to lower-left edge.

Turn off Quick mask mode - you should get nice selection similar to this.

Go to Filter menu - Blur - Lens blur. Play with Radius setting, until you like the effect.