Nerdburger said..
It's a stable board for your size, try flat water small surf to progress quicker, the above advise is on the money. Surf stance, 1/2 to 1 foot in front of other, (either side's of middle-eg handle) wide, knees bent, use you paddle as an outrigger as need. Keep moving. I'm 6ft, 90-100 kg (Xmas holidays haven't been kind, on the figure) I ride, 120 L 8'5" and ready for the next size down., 105/110 L.
It hurts some people to down size to quick, but probably not worth changing board size now, especially consider surfing back ground.
Find the right conditions to progress in will be the key, to get you comfortable on your board. All the best ??
Again, solid advice. Thanks ??