If she's stopping with your half halts, then you're using too much rein. Try just closing your fingers on the reins or steadying your hands.
To ask for more collection, try pulling your inside rein back 2-3 inches, so her head is just in a bit (should be able to see her eye). Give her more inside leg to keep her on the rail. Keep your outside rein steady. Keep the rein back and your inside leg on (squeeze or tap if you need to) until you feel her head relax and her back come up. IMMEDIATELY let up on your pressure (rein and leg) and pat/reward her, even if she only have 1/2". Keep asking like these until you get a better and more immediate response. Be sure to ask going the other direction too
Circles, figure 8s, and serpentines are also great work. Use your legs to bend her around the circle. Lift UP your inside rein (not back, just straight up) 2-3" to get a nicer bend in her neck. Keep your outside rein steady so she doesn't bulge her shoulder. LOOK UP and around your circle/figure. When you change directions, let her straighten out for 2-3 strides, then change bends.
Always remember to keep her going FORWARD. Collection must start with impulsion, which comes from the rear. If she loses impulsion, she will shorten her neck, but she will tip on the forehand and get strung out behind. True collection goes from tail to nose.
And no, it's never too late! You won't be competing in 3rd level with her (not likely anyway, lol), but you can still encourage her to use her body more effectively. She will build better muscle and get in better shape for riding, which will extend her riding career.