Hi Sophia
The first thimble I was able to use successfully was a Comfort Thimble - it is soft and flexible with a big slanted tip and lots of deep dimples - it worked pretty well. Also, I watched a tailor do a lot of mending - and something about that combination clicked for me. I assume your thimble has the outer ridge and deep dimples - so you have the right type. A quick you tube search turned up Jo Gibson - Lesson #4 big stitch quilting. She is using perle cotton but the idea is still the same. Also she uses a hoop and rubber finger to grip the needle, which I find extremely helpful. Alex Anderson uses a thimble on her index finger and ( I think) grips the needle with her middle finger and thumb. While that looks miserably inefficient to me, it would be hard to argue with someone who does beautiful work and is the head of a quilting empire. I am sympathetic to your challenge. I have never been able to make even stitches with my thumb or right hand. Just keep trying and look for a you tube demo that makes sense to you - or maybe even a live human that hand quilts - that would work too.
Lefty - in Philly