Liz -
If I read this correctly, you have the middle finger in the 3rd slide ring. So my first suggestion would be to try using your ring finger instead. That may not agree with your teacher's recommended way of holding the trumpet, but everyone is different and there is no single "proper" way to grip the trumpet.
Have you seen the ITG Youth web site? There's an article there entitled "
Get a Good Grip On Your Trumpet!" that I'll bet will help you.
If you can't find an alternative grip that works well, as a final resort you might have the ring position (assuming it's not adjustable) moved by a good instrument repair person.
I really think this is a fixable problem, and that you'll be enjoying the trumpet even more very soon. Best wishes!