If you want to train her to be a little more responsive, first ask for a halt with only your seat and/or voice (or whatever cue you want to use). Give her about 1 more stride (or however quickly you want her to respond) and then make her stop and back a few strides using the reins. You don't have to jerk the bit or use much contact at all, just whatever makes her respond immediately. Let her stand for a few moments, go back to the gait you were using before, repeat, repeat, repeat.
I used to have a very responsive mare (horseless now) and that's how I trained her. I mostly just rode for fun, too. Sometimes, to tease my Western Pleasure friends, I would ride with my reins so loose they hung down around the mare's knees. I'd have to pull up hand-over-hand three times to get contact (I was a stupid kid, I know). Even then, if I just lifted my hand a tiny bit, the horse responded. From a trot or canter, if I said whoa, she instantly dropped to a walk. I never could get an instant halt on voice alone. But, if I also lifted my hand an inch (no contact), I had an instant halt.
To train any horse (or dog, or child...) make the right thing easy and the wrong thing hard. When the horse stops immediately from the gentle cue, you never touch her mouth. When she doesn't, she has to stop and back up, too.