If you're falling off to the inside of a turn, the board isn't carrying enough inertia to complete the turn you're expecting, or you're not adding enough energy to complete the turn. Most likely, the board is stalling. The board either needs to come more up on the rail so it will turn quicker, or you need to put more energy into keeping the board moving using compression and extension of the body. When the board is starting the transition back up the wave face, you start to bend at the knees. This not only starts to "coil the spring," but un-weights the board so it doesn't slow down as much when coming to the top of the wave. About half way up, you'll start the transition to the other rail, and as it starts to come around on the top of the wave, push with the compressed legs. This imparts energy to the board (like pumping a swing), and keeps it moving. But the real trick with any turn is to be tuned into what the board is going to do, and keep your mass over the board. Keeping in mind that coming down the face into a bottom turn, the board is accelerating, and has lots of energy to carry even the straight legged through a bottom turn. But it will lose speed and energy when going back up the face. This is what makes the top turn challenging. It's either a less powerful turn, or you have to add the needed energy. Some people will use the paddle to push the board around, but paddle turns don't look (nor feel) nearly as beautiful as a rail turn

However, you can also use the paddle to impart energy to a rail turn by adding forward drive, but I don't think it adds nearly as much as driving with the legs. A lot more complex than it sounds... Keeping the mass over the board is a "feel" and practice thing that can't be taught; just comes with experience. My advice would be to just get back to the top of the wave and start by trying to turn harder and harder off of the top of the wave. Then move to start turning before you get to the top of the wave, etc. Hope that helps a little? Not a good teacher or instructor, but surfed for four decades. It's all feel, so I really have to think through the mechanics when trying to explain what I think something is, and it's not always clear...even to me

edit* Of course foot placement is also really important. Especially on a SUP. When coming to the top of a wave on a bigger board (SUP), stepping back with the rear foot can also help. There's a lot of speed coming off the bottom, so my rear foot and weight is usually quite forward to keep the board from slapping. When coming into a "lean back" turn with less speed, I find that I step back and to the inside with the rear foot. Another reason these turns are so much harder to pull off cleanly than on a short board, IMO.