Ruth is just great!
But. I think for your defense to improve - you need a stronger opponent. As I see it, the situation is like this: the topspins are slow and not very powerfull so you don't have to really chop them with a long stroke (your stroke is short, you don't use your legs and body a lot). If the quality of topspins rises you will find it hard to control returning them with such a short stroke (or you won't be able to create a good amount of underspin).
So my advice in your situation is(when you don't have a powerful looper as a practice partner): try to send the ball as low to the net as you can(this can force you to flex more.. and is always good if the ball is low), and try to be more flexible(longer stroke, more body twist, think about your legs on every chop you make) when chopping. As my trainer always says: "a chopper must be like a cat". That's just my opinion and I hope it can be useful.