Hi KiwiDave
If you struggle to self-rescue with a board in 12 knots you simply need more practice and/or a better technique, firstly without the board. You are probably trying to wrap your lines which is a difficult task on it's own. If I had to wrap my lines in low winds, I would keep the board under my feet while I wrap them as I show in this video:
Another self-rescue method which is more than twice as fast and a lot easier in stronger winds is one that doesn't involve any wrapping, but for this one I prefer keeping my board afloat and pushing it in front of me as I pull myself up the safety line. No kicking and no swimming = no tangles
If the board is too much of a concern during the self-rescue think about letting go of it and self-rescue as fast as possible, this means you get back to the shore quicker and have more time to look for the board and more time to signal other kiters to get the board for you before it drifts too far downwind.
Most kiters aren't very competent at self-rescue simply because it's not taught well enough nor is practiced well enough to ensure efficient results when a real situation occurs.
Were you asked to do a solo self-rescue in deep water in your lessons or did you just get a demonstration/simulation with the instructor standing on the beach or in shallow waters?
Christian