It was a good feeling for me when I finally learned some techniques on how to deal with someone on my back. Before learning anything I would quickly be tapped out once in this position.
In Kurt Osiander’s video, he describes the fundamentals of keeping your chin down and protecting your neck. IMO, this is the first priority, to fight to defend your neck from the choke.
I like how the next video below describes moving your hips and getting your back to the mat to escape. There are many variations around grip fighting when defending your neck. When possible, I usually try to do the two on one hand grab on their choking arm to move my head to the outside. If I can successfully do this and keep control of their arm, I feel way more confident.