Best board to learn and progress on?

[NorthVegas14]

I took my lessons on a Slingshot Fuel 15 and a 190cm very floaty board (I can't remember the brand or style, but my guess is the manufacturer only sells them to schools) on a day when it was blowing about 16-19 mph. I was able to waterstart and ride that board on my first try with the board during my lesson. Of course the instructor was kneeling in the water in front of me holding the board steady while I dove the kite.

I'm a 6' tall male that weighs 165 lbs. After getting up on that board on the first try and riding away a good distance on my good side, I fell in love with this sport. I immediately bought some equipment that, given my windsurfing and snowboarding experience, the instructor/salesman thought was appropriate for me. It was a North Vegas 14 (2005) and a North Dragon 138 board. I love the kite (a little difficult to water re-launch sometimes), but I'm afraid I may have gone too small on the board, even though it is classified as a light wind board. I have gone out on it in overpowered conditions (19-21mph) once and had no problem getting up on it. But in 13-16 mph conditions (which should be ideal conditions for this kite for a novice), I can't seem to do anything but round up on it the second I get up, which kills the kites power and lowers me back into the water.

I didn't want to get a "really big" board since I thought I would outgrow it almost immediately. Now however I find myself going nowhere in my learning curve and I'm wondering if that would change if I bit the bullet and just bought a much bigger board. I'm thinking in the 160-162cm range. The North Sumo 162 (2006) board seems like it may have been a better choice for me, but at $599-649, it seems like a waste to get a board that I'll never use again once I get over the hump and it all clicks.

Any suggestions? Should I just keep trying to progress more on the 138, or is that a hopeless cause? If so, what board might be a good alternative? I was also thinking maybe the Jimmy Lewis III in the 145cm, but it seems too close in size to my 138. Any help on what board others have used to get them from a beginner to the next level would be greatly appreciated.