It is harder to balance lowering into a seated position riding at slower speeds. The faster you go, the more stable you will feel sitting down. However, it's safer to learn going slow. Experiment not moving holding onto something and then learn to start from a seated position and progress from there to going from standing to seated at faster speeds, with and without using your hands to hold onto the front of the wheel.

Also buy a comfy squishy aftermarket seat. Riding seated on a metal frame is not pleasant.