How to stay balanced in the canter

I observed last week that while I was cantering, my butt was going here and there and I thought I was going to fall off. It wasn’t the front or the back, it was the side that I was concerned with.

I had a thought about it, and I remembered Yannis telling me that there’s a problem with my balance. So today I searched on YouTube for videos focusing on keeping oneself balanced while cantering. And I found the one that follows:

The lovely lady here says that stomach is very important. The problem is that, I have never taken my stomach into account when it comes to riding!