Can I suggest you use a guitar strap? You know, just a standard shoulder strap. It might help you stabilise your guitar. I mention this because you said you felt like you lost control of your guitar when you attempted to move your thumb position. I guess most people think you only need a strap when you are playing while standing.

I think a guitar strap can help with keeping the neck from moving away from your body, essentially adding some resistance to your thumb pressure, especially when you are starting out with guitar.

Might not help your situation, but maybe worth a try.

Good luck!