I'm 6'5 and I learned to skate on a pennyboard. I loved it so much I use it to commute to school everyday with a heavy backpack. I live in Oregon so the weather is always wet. In two years I haven't had any serious injuries. Obviously, I'm probably stupid for not buying a bigger board but I just can't get over how much fun a 22' deck is.

From my experience, it sounds like you are just too stiff or nervous when you ride. Try to relax and just keep trying to ride longer and longer on a flat surface. The important part is finding your balance on the board. You can't really understand the limits of this balance unless you keep falling off the board, so keep trying! Once you find this balance, you can fluidly move your body with the movements of the board. Worse case if you feel the nickel is too slippery to ride, buy some grip tape from Amazon, it's not expensive. But I would recommend you learn to comfortably ride first, so that you can tell the difference with the grip tape on.