Trouble balancing when starting
The more speed you get, the more balanced you will be, so at the beginning give a hard pedal stroke to get quickly some speed. You can also practice in a slight downhill road, this gives you speed easier.

I recently taught a friend to ride a bicycle (30 years old!), and the solution was the slight downhill.

Another trick is to look far ahead (dont look at your front wheel) and stay upright, so you should put your saddle higher than you describe.

Good luck!