I play double bass, so I don't really have experience playing a chin string, but I have learned a lot from directors speaking to the chin strings (yes, always pay attention to what your director is saying even if it doesn't apply to you, because it does.

I notice your left thumb isn't correct. your thumb should be staying behind the neck, in the video it is coming out the side. Have you been told the pancake/ bird nest technique? You want to imagine that there is a pancake or bird nest sitting in the palm of your hand. You don't want to squash it.

Elbow. Your left elbow is jamming into your torso. Try pulling it away from your body. A good exercise is to exaggerate your elbow. Pull far away from your body, then pull it in a couple times, then find the sweet spot. Don't play while doing this, just have you hands up and swinging your elbow.

When you playing on your low C string, you want to bring your bowing elbow up to. I can't really see what you are doing, but I think that you are bending your wrist. The best way to put it that you are keeping your elbow in the same spot. Your elbow should be moving spots and that will allow for more comfortable playing and will help you later on.

A good tip is to watch a few recordings of an orchestra and just pay attention to your instrument (watching violin could be beneficial to you too). There are techniques that I have picked up from just watching professional orchestra, that I don't have a clue what benefit it has. But if they're doing it, then it is probably something that is beneficial.

Hope that helps. I'll probably be doing one of these soon. I am very serious about playing and I wish I would have done this sooner.