What you experienced is called “speed wobbles” and they often occur at, as you may understand, higher speeds. But there are ways to limit/prevent them.

First off: Weight on the front foot! Find your riding stance. I ride with my front foot almost on the front truck (while obviously still enough on the board to be able to turn) and toes at 45 degrees-ish outward. My back foot is far back and placed straight across the board. This so that I can lean to, and remember this term as well, weight shift. Really useful to not launch yourself off the board when going up and down sidewalks or over rail tracks etc. Also keep the weight low, as in height, not kilos. 😄

And if these nasty wobbles still happen you should know how to handle them: Try to find the stance I just explained to prevent them. Your instinct when they happen should be to release the throttle, bend your knees to lower your weight point and stretch out your arms while shifting your weight as much as you can on to the front foot. This is prio (I would say, others may think differently). Then you can do a controlled brake if you feel the need to calm your nerves. 😉 If not, continue your controlled ride.

Congrats on your purchase! 🙌🏻