There is a tutorial and a script to automate making seamless tiles here...

http://www.cartographersguild.com/showthread.php?t=1373

The problem that you have is that the overall contrast of the tile is too high and it needs 'flattening'. Using the script there are two parameters, one is the flattening amount 1 - 3. I would go for 2 unless you think otherwise. And the second parameter is the image to tile and flatten.

To break up repeating patterns of a seamless and flat tile. Take a layer and flood it with the tile as you have already done. Then take another layer and flood it with the same tile but stretched a bit in both directions. Resample it maybe 131% or so and then flood it into layer 2. Then blend both layers together. Prefereably with a nice noise function but you can do a straight 50:50 average and it still looks good.